Really need an expert for this speed issue

Ok, first problem is, I have that same exact router, excellent router, multi gig, if you read the routers documentation, it states it has a 2.5 gig Ethernet port, if you read farther it states it can used on the wan or lan side, not both at the same time, so essentially limited your speed to 1000  Mbps via Ethernet connection, there are ways around it, but see your problem

So check out link speed negotiation, if your device cannot negotiate the higher speed for some reason, it will default to the next lower speed, which is 1000 Mbps

Yes, exactly, for some odd reason i was never able to identify, my network card was never able to negotiate the fastest link speed, defaulting to 1000 Mbps, i found if i manually set the the speed and duplex to 2500 Mbps, full duplex , which my card supports, instead of auto, it worked as expected, considering the tech brought his own equipment, same results, i don't know, but i would defiantly try this to positively rule it, in or out,     

Speed and Duplex: Just as the name suggests, this allows for selecting the desired speed and duplex of the network adapter, the default setting is usually auto negotiation. It is recommended to set the highest speed your adapter supports (Gigabit, if supported), full duplex (two-way simultaneous communication), and auto-negotiation (enabling the adapter to negotiate the highest possible reliable speed). It may sometimes be necessary/helpful to manually set the adapter speed instead of using auto-negotiation.

https://www.speedguide.net/articles/network-adapter-optimization-3449 some other settings that might be helpful

I have tried that and it didn't change the results. It really seems like there is an artificial limit being set my XFinity.

Just curious what type of network card you have for your computer, that supports over 1000 Mbps, Usb, internal? 

Internal built into mother board. ASUS TUF GAMING X570 Pro. 2.5 GB Ethernet.

Ok, thanks, I have the same router, rt-ax86u, I get this this all day long

Did you try to get the drivers for that network card asus website? 

Yes I have done that. Very impressive speeds. 

Ok, ill tell you a test i have been doing for decades, its very unconventional, but cuts to the chase very quickly, right now you have too many variables that could be suppressing your speed, you seem to have tried everything, so my computers are multi boot, i have several operating systems on each one, Win 10, Linux, FreeBSD etc, so if I have a perplexing problem i cant get a handle on,  i think to myself, maybe its the operating system itself causing the problem, so i boot into Linux mostly, because it just Works, like in your case, while in Linux i would do speed tests etc, being in Linux it isolates the Windows Operating System completely out of play, so i am on the same computer, exact same connection, exact same modem/gateway, exactly all the same hardware, so now i do a speed test etc, so if you get the correct speeds, it rules out everything except the Windows Operating System, so it greatly narrows down where to spend your trouble shooting efforts, so you can do this same test by downloading the Free Ubuntu Linux install iso, so put it on a flash drive, boot your computer from it, it boots into memory, have your ethernet cable connected, its boots into a very fully functional desktop, you will be online with no configuration on your part, it gives 2 options, install or try out, obliviously try out, Firefox will already be installed, so you can do some speed tests etc, analyze your results, personally i always find it has something to do with Windows in most cases, if you get the same speeds it rules out Windows most likely, like i stated unconventional, but if you tried everything else....

Not a bad idea. But when I boot up modem with router connected using 2.5gb port, I am not using windows at all. My phone is wifi 6 and I have had 2 times when I had over 1300 speed using my phone. Then it jumps back down to the "artifical limit". Never do I get 1050 speed, or 1140 speed or 1205 speed or even 975 speed.

The time I got 1420 speed on my computer it was days of 850-950 speed. Then they added my modem again after taking it off my account and bam, speed goes up 40% to only go back down a few mins later. Nothing changed in windows. Just seems super unlikely it would be that. Same limit on another computer and the tech computer using his Sb7 modem. 

@GoSteelersGo But when I boot up modem with router connected using 2.5gb port, so if you connect your computer to the router, you are using the 1000 Mbps port, capping your speed to 1000 Mbps, that router can only have the 2.5 gig port, wan or lan, not both simultaneously, i have found ways around it obviously,  @GoSteelersGo Never do I get 1050 speed, or 1140 speed or 1205 speed or even 975 speed, the math tells me its defaulting to 1000 Mbps, on wifi i have never gotten over 900 Mbps, that was 4 feet from the router, also the techs network card might have been a gigabit, limiting it to 1000 Mbps

When I hook to computer I used the 2.5gb port. Yes if I use the other port I wouldn't get and couldn't get 1400 speed of course. 

When I got 1300 speed from my router I switched ports and gave full 2.5 to router. My phone is S21 Ultra which is wifi 6 and can handle that speed. Not sure what phone you used. 

1. @GoSteelersGo The supervisor checked the speed with his tester at the street level and was getting a little over 1300. He tested with his tester coming right out of my wall, got the same speed 1300, that there to me tells me the problem is in your hardware, or a configuration issue, I tried to explain to you some tools I use, I tried to explain about that router, I have the exact same model, I even go so far to compile my own custom firmware for that router, but you want to dismiss my ideas as highly unlikely, it’s ok, I get 1500 Mbps and you dont , I am done here

Going from the cable modem directly to the router bypasses all windows. I get what your saying about your idea. I don't have the same level of expertise as you to try to figure out another operating system. Since the same thing has happened using cable modem to router as using a windows computer, it doesn't seem like it's a windows issue. I can see if someone can help my try your Linux idea. I am open to trying anything. But I need to be able to do it. 

First i want to apologize i was harsh, i received some bad news,

This the Linux desktop you would automatically boot into, on the left you see the familiar Firefox icon, its point and click like windows, i wouldn't suggest things that i didn't folks couldn't actually do, with no configuration on your part you get this, Linux has come along ways, first off remove the router from the equation, trust me on this, if you connect to the router via ethernet cable, you wont experience speeds about 1000 Mbps, i just tried to post a very long and detailed explanation, but i wasn't  at home, i took my dog swimming and it didn't post, its not so much about windows or your other device, its using a different operating system with different drivers, Linux just Works for me, i been using it since 1997, so its my go to trouble shooter, the speed test can tell you if your equipment can actually handle the increased speed, if you get results faster and correct for your speed tier, then it obviously can, if not maybe your equipment cant, so since your equipment never experienced faster and reliable speeds, a couple random speeds tests that you did, that you can't duplicate in 2 weeks time, i find suspect, when i trouble shoot i explore every suggestion, hint etc, even if i know its probably not the issue, because the person suggesting it could possibly know something i don't, Murphy's law will see to that, so let me recap, take the router out of the mix, i spent considerable time experimenting before i realized this fact, that router has a option called "Dual Wan" that's what i use to get my 1500 Mbps speeds, even if you don't believe me, refer to my trouble shooting philosophy, when i got the speed increase, i didn't get the faster speeds either, but i worked thru it methodically....