Can I create a guest wifi login?

Unfortunately you can do so right now!   The web GUI and admin functionality of the 5G modem is severely limited. WYSIWYG.   I have asked support, and submitted lots of suggestions.  However, many voices get things done, I would recommend that you call and voice a suggestion/compliant that you want more robust admin functions for the LOCAL side of the network on that modem.

Just about every other ISP and their modems have these features; not sure why they were turned on the T-Mobile modem.