Worst possible customer service -- internet out for two days, zero response

Comcast / Xfinity seems incapable of even recognizing my (85-year-old neighbor's) problem, and she is without internet with ZERO response from Comcast.  It's unbelievable.

Here is the story:

I helped her replace her broken Technicolor CGM4331COM gateway two days ago -- it was incapable of giving out reliable DHCP addresses, and devices would suddenly become "reserved" but disconnected after a few days.  Rebooting the gateway was required to get it back.  This is a clear problem with these gateways, I see it all over these forums.  So we get her an Arris G20 instead.

The changeover went absolutely swimmingly -- after installing the app on her phone (and ensuring she had good cellular data), we were able to take the QR code shot of the MAC address on the new modem/router, and it transferred over perfectly.  Internet was just as fast, and all devices connected fine.  Everything worked for about 16 hours.

The next day (yesterday) she went to the Xfinity store to return her old Technicolor gateway.  I did not accompany her, but for some reason the agent at the store was asking her for the serial number of the new gateway; I don't know why, and of course she didn't have it (nor is there any reason why they would need it).  Anyway, by the end of her visit, the agent told her she was "good to go" (his words exactly).

When she got home an hour later, her internet was down.  Then, it came back up.  For the last 30 hours, it has been up for about 10 minutes at a time, then the Arris reboots.  It comes back, stays on for about 10m, then reboots again.  Reboots take about 5m each.

I went to her house this afternoon to try and get the problem addressed.  Xfinity is incapable of allowing me to describe what the problem is!  Every time you call or use the app, they start  by rebooting the modem.  Of course, it comes back after 5m, and is working perfectly well -- and that's the end of support!  Nothing seems broken, so they just disconnect the app and/or the phone call, and you're done.  Zero ability to contact anyone.

At one point, after screaming "Agent! Agent!" into the phone enough times, I got a human on the line.  What did they do?  Reset the modem.  We waited for it to come back, which it did.  I told them that it will be out again within 10 minutes.  They promised they would call back and check.  Never did.

I was able to get an agent one more time after several phone calls -- each of which required waiting for a modem reset, then waiting for it to come back -- and this agent ran some new tests.  He said he would call back within 30m to one hour.  Never got a call back.

So now her modem/router is rebooting every 15m, and every call/app contact made to Xfinity is the same:  "I'll reboot your modem, and you can call back if there is a problem" and it just tries rebooting the modem again.  One time I was able to force the call to stay until the modem reset, but of course, everything was working when it came back -- and then, 10m later, it rebooted again.

Again, there is no process that will let me explain to Xfinity "the modem is rebooting every 5m, no amount of your or my rebooting is fixing this problem".  It's just a black hole.

Maybe this brand-new Arris gateway went bad after 16 hours, and now works for 10m and then resets itself.  But is it just coincidence that it started happening right after she visited the Xfinity store and the agent wanted a serial number?

I think the next step is going to be to disconnect the cable from router, go through the app and make them believe that the internet is just plain out.  Maybe then they'll do something.

It will give me time to file a complaint with the FCC.