i am no expert in this field but here is my take
i used google dns and on sundays, i find that i am not able to surf. after resetting to use back the default dns in the router, everything goes back to normal.
tm is using dns to block sites, and to control this, they tell users that only the modem and routers that supply can be used for unifi.
notice how they dont provide you with your xxx@unifi password? maybe to prevent you from setting up unifi on another router?
did some reading and they probably use tr069 to remotely change your dns as in above replies.
probably use this tr069 to block you from surfing if you are using another dns server.

i dont know man.