Nintendo Switch "NT Traversal process has failed"

This error just started appearing tonight. I have been successfully playing online since I first signed-up for TMHI, now all of a sudden (BOOM!) I can't connect to anyone, nor can they connect to me.

I've restarted my gateway (Nokia) and Switch multiple times to no avail -- I've also tried using custom DNS, with no improvement. I also tried connecting to my mobile hotspot (also T-Mobile) which worked fine before -- now nothing.

My NAT type as reported by the Switch is “B” which should be good for connecting (It’s always been B, when I was gaming fine before)

Did T-Mobile quietly change something which kills Nintendo Switch Online? Are there any solutions which don't require me to add yet another monthly subscription to my growing list?

Nintendo Switch Online is an important part of staying connected with distant family members, so any help is appreciated.