Internet connection problems becoming more frequent and persistent

Over the past 6-7 months, our internet connection has become less and less reliable. Whereas back then we'd lose our connection (or get a very slow connection) maybe three or four times a month, now we're at the point where we rarely have a stable connection for more than 10 minutes at any time on any given day. We've had Cox for 10.5 years. The service has generally been reliable up until recently. Now we can barely be on line for more than a few minutes before the connection is dropped - and refuses to come back on until hours later (if we're lucky!).

I replaced our router about 3-4 months ago; upgraded to a TP-Link AC1750, but that didn't help

I replaced our modem last week; upgraded to a Arris SB6190; but that hasn't helped, either.

Yesterday I replaced the coax line from the wall to the modem and the Ethernet line from the modem to the router; the problem persists.

We seem to have good connections between our devices and the router, so I'm ruling out the router as the source of the problem. And now with a brand new modem, I'm ruling that out, too.

Any other thoughts or suggestions?

We need HELP now in a big way since both my wife and I rely on our internet connection to do our work from home!