How to use High-Speed Internet Gateway with Existing Router

Sadly, putting your original router into AP or bridge mode doesn't solve the problem very well, since most people have lots of networked gear already installed and configured - TV's, thermostats, cameras, Alexa's, lightbulbs and hubs, gaming consoles, etc etc.  Requiring all of those to be reconfigured to use TMobile as the authorative source for IP addresses and firewall rules (much of which cannot be adjusted using their simplified version of the controls) just isn't workable. To say nothing of range-extenders or meshed APs - all of which are common for the kind of customers who need 5G internet connection.
T-Mobile needs to keep iterating on the firmware here until they get this right for all their customers. Fingers crossed.