Trouble with connection to work VPN

Here was my situation and how it was eventually fixed.

We've had Spectrum Internet for a few years and everything worked great but they kept increasing the price on us. No problems at all with anyone's access. We switched to TMobile Internet right after Thanksgiving (Black Friday deal) and the modem wouldn't work right out  of the gate. Took almost a month for them ( and 2 SIM card changes + 1 modem change) to figure out that it was a configuration issue on their end. So had the system up and running right before Christmas and it worked great for everyone. But then, right after the holidays were over, my wife fired up her work computer for the first time on TMobile Internet service and right away had the "connection lost" issue every one describes above. She would start up and connect but then, after some time, Outlook, Teams and Edge would all show as disconnected. She could switch to another router or even ethernet and this would temporarily fix her connection but it would always come back. I had been working in the meantime on my work computer without any issues on both the Spectrum and TMobile networks. I tried all of the fixes mentioned above but none of them worked for her. Finally got another modem, plugged in the old SIM card and voila it worked immediately. There were 2 other symptoms of this that also went away - my Ooma VOIP system reconnected (it had been flashing red for weeks) and my Ring cameras stopped acting flaky when trying to view video.

Here are the OLD (non-working) and NEW (working internet) modems. So looks like its a modem issue