T-Mobile Home Internet Slow IPv4

Not encrypting DNS... seems like overkill when I am letting the routers forward DNS queries to TMO's v6 servers instead of trying to force external public DNS servers.  Keeping it "in-house" and not reaching across the internet to 3rd party DNS servers... which is how TMO seems to want it to be done.

I only set alt DNS entries up at the bottom of the list on the laptops for when I am not on my home network... but even then, I often use a VPN and lock that down against local LAN access and such anyway.

Seeing as things go sideways when you force the encryption (strict) versus opportunistic (tries, but falls back if unsupported)... sounds like TMO's topology up stream may again be getting in the way because you are continuing to deviate from their program.

Keep in mind that all your packets are going through additional encapsulation to go across TMO's networks... why the base MTU is lowered to 1420 instead of the usual 1500.

Have you tested it with the encryption disabled and running primarily against TMO's DNS as described earlier?