Best Configuration for a Mesh Network Connected to the T-Mobile Internet Device?

I have the same problem as cjb4 plus the xt8 drops the connection frequently; after researching it on the internet, found these possible causes.

1. Double NAT: Since T-Mobile's Home Internet likely uses its own router, connecting your ZenWifi XT8 to it creates a double NAT (Network Address Translation) scenario which can cause connectivity problems. This can be mitigated by putting the T-Mobile device in bridge mode (if supported) or disabling its wifi entirely.
2. MTU mismatch: T-Mobile's network might have a different maximum MTU (Maximum Transmission Unit) size than the default setting on your ZenWifi XT8. Try lowering the MTU on your ZenWifi XT8 to match T-Mobile's network (reported to be 1420). MTU: The Maximum Transmission Unit(MTU) of PPPoE packet. The default value is 1492
3. IPv6 issues: There have been reports of trouble with the ZenWifi XT8 getting an IPv6 address from T-Mobile's network. Asuswrt-Merlin firmware may offer more configuration options for IPv6 than the stock Asus firmware.  

I tried disabling the WiFi on the gateway, it did not fix the issue.  I think the issue is #2.  Asus software does not have the ability to change the MTU value.  Asuswrt-Merlin firmware does but I couldn't find a version specific to the XT8.

#3 I turned off IP V6 in the adapter settings there is still significant performance drop 170Dn 16Up.  When I connect to the XT8 router using ethernet cable the performance is terrific 360Dn 15Up