Short to very-short DHCP leases

I'm hoping someone can help me resolve my internet problem: For the past several weeks the Comcast servers have been giving my router short to very-short DHCP leases.

For years, my typical lease time had been several days. But about 2 weeks ago, the leases have been much shorter, usually 1 or 2 hours, sometimes as short as 20 seconds.

Each 1 or 2 hour lease renewal causes a brief drop in my internet, a 20 second lease confuses my router and causes the router to restart.

I've had multiple, deeply frustrating calls with Comcast support over the last 2 weeks. Suffice to say Comcast support has been considerably less than helpful.

Giving up on Comcast support, I then scheduled a tech to come to our house. The tech said the modem, router, wiring & signal levels are ok.

The tech observed the short-lease problem, examined the router log, then called his supervisor. The supervisor likewise examined the log,

and said the problem was likely on the Comcast server side and he said he'd submit a "network ticket" for the "backend team" to check the

DHCP server settings, CMTS logs and provisioning. The supervisor assured me the backend team would call me right away and would send me updates:

However, I was never called, I've gotten no updates, and the problem persists.

More details:
The modem is an Arris S33, the router is a TP-Link BE230.
The short-lease problem persists if I swap out the router for a laptop.

The short-lease problem persists if I swap out the Arris S33 modem for an older Motorola SB6121 modem.

I don't know what to do, I've run out of options. Comcast seems to have a problem at their servers, but Comcast isn't doing anything about it.

Meanwhile for weeks I've had an erratic internet, with little likelihood it will improve.

Searching posts on this forum, I see short leases seem a common problem with no definite solution; I've tried the suggested solutions to no avail.

I'd be very grateful if someone could advise me on what I can do to fix this and get back multi-day DHCP leases.