Not receiving advertised upload speeds

This may or may not be the root cause of the problem (YMMV) but it needs to be addressed regardless. 

The upstream power is way too high / out of spec. That can cause random disconnects, spontaneous re-booting of the modem, speed, packet loss, latency problems, and the un-bonding of channels.

In an effort to try to obtain better connectivity / more wiggle room, check to see if there are any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/re-configured. Any splitters that remain should be high quality and cable rated for 5-1002 MHz, bi-directional, and no gold colored garbage from Home Depot, Target, Wal-Mart, etc. Splitters should be swapped with known to be good / new ones to test.

Also check the coax cable for any damage such as cuts, nicks, abrasions, kinks, sharp bends, etc.

If there aren't any unneeded splitters that can be eliminated and if your coax wiring setup can't be reconfigured so that there is a single two-way splitter connected directly off of the drop from the street / pole with one port feeding the modem and the other port feeding the rest of the house/equipment with additional splits as needed and you've checked all the wiring and fittings for integrity and tightness and refresh them by taking them apart then check for and clean off any corrosion / oxidation on the center wire and put them back together again, then perhaps it's best to book a tech visit to investigate and correct.

Good luck with it !

@user_kmh717  

That modem is not compatible with the Next Gen speeds or one or two hundred Mbps. Only these are;

https://assets.xfinity.com/assets/dotcom/projects/cix-4997_compatible-devices/2024.05.17%20Full%20List%20of%20Compatible%20Devices.pdf

See the top table that says "Next Gen Speed Tier".

@EG, 

Thanks for the reply.  As for the speed question, is the new upload standard proprietary and not compatible with vanilla DOCSIS 3.1?  My modem is capable of 8 upstream channels.  

- Matt

Quite welcome. Other than that table of third-party retail modems, their latest rented gateway devices (XB6, 7, and 8) are also compatible.