My experience with the new T-Mobile Home Internet (NOK5G21)

It's setup and configured as a Tri-band router and supports WiFi6 and mesh.  Take the time to set it up so it works for your networking needs.  Take it out of auto channel selection and use the channels that work for you if you need to.  IF you use it in tri-band mode, Roku's, your phones and most devices including WiFi6 capability will handshake and switch between the 3 bands seamlessly.  The gateway will also load level between the 5GHz low band and the 5GHz high band automatically. Be aware that the auto channel selection picks DFS channels for the 5GHz bands and ROKU's won't connect to those.

The tri-band works on having SSID1, SSID5 and SSID9 all configured to the same SSID name and encryption mode.  

SSID1 is the 2.4GHz

SSID5 is 5GHz low band

SSID9 is 5GHz high band

Use WPA2/WPA3 and AES for all 3 to avoid any issues with the auto band switching.

Note that if you use the other SSID's 2-4, 6-8, 10-12 you end up with a bunch of SSID's on the same channels and there is no guest mode or SSID isolation so pretty useless. You can turn off the individual radios but that requires every SSID to be set to enable SSID and broadcast SSID to off, all 12.

If you don't want to use it this way then pick unique SSID names for each band and connect your devices uniquely to that SSID so they don't automatically switch and potentially drop connections and hang. None of my connected devices have issues with the auto switching but yours might.

Cheers