Setting up guest network on t-mobile highspeed home internet

An update: We bought a Linksys AX1500 and it seems to be working great. Regular routers didn't seem to work -- we tried both our old one and a new one without success. The mesh seems to somehow make the difference. We set up a guest network, and so far so good on getting all the IoT gadgets on it. As always a key thing is to have the gadget on the same network as the phone used to set them up. Follow-up questions: Can we/should we delete the SSIDs/2 networks on the T-mobile high-speed device, leaving it to function just as a modem? Are there pros/cons to doing that? And is there a way to change the user name of the t-mobile admin page? I see where I can change the password but can't see where to change the "admin" user name to something else. Thanks again for all the help!