After 3 months of trying to get speed issues resolved, a tech visit only confused things. Help!

Backstory: In June, I noticed that things seemed off with my service. A speedtest revealed that I was getting around 50-70 Mbps down and 25 up. I pay for 900 down. 

Went through the usual suspects, rebooted my modem, my router, tried the speedtest directly from the modem. No Improvement. For reference, I own an Arris S33 Docsis 3.1 more than 

capable for the correct speeds, and when I first bought the modem (to go with the service) it worked fine. 

Called tech support. More rebooting, waiting on hold, etc. Xfinity chooses to send a tech out to take a look at things. The tech checks the modem, the router, and finds no problems. He says he thinks there is something

wrong with the line, so he runs a new line from the house to the box. No changes. Same slow speed. He says the problem must lie in the line between the outside service box and the 'pole' (which in this case I believe is underground at the end of the street). He sets up an appointment for another tech to come out to deal with that. 

Well, that tech shows up unannounced when we are home, fails to knock on the door, and just runs another new line from the box to the house (I see this later on security footage). 

When I call Xfinity, they state that in a week or two, someone else will come out and mark the road, to see the underground lines, and fix the line. I'm very busy at work and put the issue on the back burner. 

Fast forward to the last two weeks. I begin again making calls to Xfinity to try and resolve the problem. More rebooting, etc. They say my modem has rebooted itself 181 times in the last month. I don't reboot my modem often, so they make me call Arris and talk to their support, who examines the modem and doesn't think that there is a problem. He thinks that it's working fine, but that he thinks there might possibly be a problem at the pole, because the upstream channels on the modem show too low of a power rating. I call Xfinity back and they schedule a tech. 

Today, Xfinity sent another tech to the house. He examines the house, finds no issue with the router. He finds no issue with the modem, but just to check, he plugs in a modem he brought with him and it has no speed problems: 1300 Mbps down! He also says that he sees some issues with the exterior lines, and will schedule another appointment to try and fix that. 

At this point, I stop by walmart on the way home from work and purchase a brand new S33 Arris modem. I figure that my modem is bad, it must have been damaged by lightning or something in some subtle way, and that a new one will solve the issue, because the tech was able to get fast speeds from the modem which he brought in. I plug in and activate the new modem with the help of Xfinity tech support. 

The new modem is experiencing exactly the same issue as the old one: Slow speed, all speed tests are limited to around 40-70 Mbps download speeds. 

I'm extremely confused at this point. Is there some issue with the S33 Arris modem and Xfinity service? It's listed as a fully compatible device! Is there some throttling taking place? Is Xfinity trying to get me to buy their modem(which I actually would refuse to do)?

I build computers, do light coding, run multiple servers in my house which I built like a minecraft server for the kids, and a NAS and wicked gaming machine for myself and my family, and do light coding. I'm fairly good with this stuff, but apparently this one is beyond my capacity to understand. 

I'm considering buying a netgear modem online which is on the compatible list and trying that, but other than that I'm out of ideas. What [Edited: "Language"] is going on?????