Paying for 1.2Gps Xfinity plan only getting 250Mbps

@Jlavaseur​ Yes!  An excellent point!

Will have to explore this. 

Updated drivers; no change

@flatlander3​ Interesting you say that, there is a splitter at the main input, however, the tech stated that if the main cable is hooked directly in the modem, the modem cannot handle the egress signal; or something to that effect, nonetheless, I did bypass the splitter and right into the modem; same results. He stated that the splitter provided "signal balance" 

Not to muddy the waters, but these results are from me standing in front of the router speed testing on a iPhone 11 Max today. Wireless faster than wired. 

@user_648a1d​ No, that's a good data point, not muddy at all.  So the WAN connection CAN work fine.

What does the power level on the line say?  That will tell you if you need the 5.5dB forward path attenuator drop or not.  Channels locked?   Any errors?  Might look just fine since your iPhone gets data OK.  I'd still look.

I do like @Jlavaseur 's boot it with Linux theory.  Maybe make an Ubuntu USB stick and boot it -- you don't have to install, it runs as a live boot disk.  Worth a try.  22.04LTS is as good as any and https://ubuntu.com/tutorials/install-ubuntu-desktop#3-create-a-bootable-usb-stick

@flatlander3​ See below power levels. I hope this makes more sense to you than me. This is over my head at this point. Keep in mind, the tech and I tested from the same modem using the 1gig ports; he was getting 800-900 on his test device box, while I was getting 250-300 on my laptop with a 2.5gig ethernet card. I just don't get it! 

1 429 6.300003 40.366287 QAM256 Locked
2 435 6.199997 40.946209 QAM256 Locked
3 441 6.300003 40.946209 QAM256 Locked
4 447 6.199997 40.366287 QAM256 Locked
5 453 6.599998 40.366287 QAM256 Locked
6 459 6.599998 40.946209 QAM256 Locked
7 465 6.599998 40.366287 QAM256 Locked
8 471 6.800003 40.946209 QAM256 Locked
9 477 6.800003 40.946209 QAM256 Locked
10 483 6.800003 40.366287 QAM256 Locked

upstream
5 35.6 41.520599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked
6 40.4 42.260300 US_TYPE_ATDMA 2560 KSym/sec 64QAM Locked
4 29.2 41.770599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked
3 22.8 41.770599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked
2 16.4 41.770599 US_TYPE_ATDMA 5120 KSym/sec 64QAM Locked

@user_648a1d​  You're running towards the top side on down stream power, so leave the 5.5 db attenuator splitter, otherwise you'll overshoot (rebooting issues then).  iPhone is getting in and out fine through WAN, probably not line related.  Upstream looks erratic, but maybe that was just right then.  Shouldn't be all over the map on power output like that.

800-900 -- probably a 1Gbps port on the tech's test box.  You can expect 850 or more, but likely less than 900mbps usually on 1G Ethernet links due to driver/protocol overhead.  That's pretty normal.

I think you're back to Windows drivers.  Get a chance to boot a Linux CD/USB stick?  If anything, it might not know exactly what the chipset is but may run at least 1G.  Or it may, and be fantastic.  Ubuntu usually stays fairly current with hardware believe it or not. 

Just want to see if it's better or not than what you get with windows.

@flatlander3​

Upstream looks erratic, but maybe that was just right then.  Shouldn't be all over the map on power output like that.

FWIW. I see 41 to 42 dB power level values for their upstream channels ?