I'm switching from Xfinity and still have both network but my windows 11 laptop says can't use T-Mobile WiFi

Since it is a Windows client make sure there are no pending updates lurking in the wing. I find the Alienware machine on W10 can act strange when Microsoft has pushed out an update and it needs to have a reboot, which has not taken place. If the client is only a few years old I would guess it is at least an 802.11ac network adapter. To be sure you can confirm the adapter capability with the system information reporting. My guess is that it probably has to do with the authentication type. T-Mobile seems to have WPA2/WPA3 by default. A client that is only a few years old should have no issue with WPA2. 

To confirm the IP address you have with that adapter you can use the GUI via device manager or whatever it is called in W11 or use the command line. If you issue the command ipconfig /all | más you can then use the space bar to page down through the network information and see the details for the network adapter addressing. You could also issue the command ipconifg /release to have it let go of the ip address it has and then ipconfig /renew to have it request a new address via DHCP. The client will not request an IP address on the wireless network until after the authentication process to begin with. If the client joins the network but does not get an IP address I would agree it is possible a static IP address was assigned and needs to be changed for dynamic addressing.