How to reach an actual knowledgeable support agent

I am having a REAL bad time while trying to interact with comcast support. 

Attempting to chat gives an error that my current user role doesn't have access to billing information (it does, I am a manager of the account with what should be full access).

Attempting to call sends me to what I assume is an overflow call center where somehow it's easier to hear the people shouting in the background than it is to hear the under-trained service rep.

The Actual Problem:

About a month ago I upgraded my internet service to a higher speed tier, and in order to use that speed I had to buy a new modem.  I refered to the list of supported hardware at https://www.xfinity.com/support/devices/ and selected a Motorola MB8600.  According to instructions, I should be able to activate on my own at http://xfinity.com/activate  -- this was not possible, even though my router is showing a valid WAN IP.  So I called to activate over the phone (a call that took much longer than it should have, previously this process took about 5 minutes and this time it took an hour and a half).

Modem rebooted to the same state, address given, no traffic passing.  By this time others in the household are FLIPPING OUT, so I give up and connect my old modem which connects in under 30 seconds.  The next  available window to work on this is spent on a call to the overflow call center where the service rep is telling me my old modem (SB6121) is not compatible with my speed tier (I KNOW) and to get a compatible modem (I already did...) or lease a comcast internet gateway.  I think this is the rub.  I think that comcast is purposely making third party equipment a horrible experience because leasing equipment is perpetual income.

So who do I call?  Who can I email that is capable of ACTUALLY troubleshooting and verifying that my new equipment is correctly authorized on my account?