Connect to the modem with an ethernet cable or over the Wi-Fi network. You don't need internet access because you are only going to be logging into the modem. You can use a cell phone, but it is more difficult because you will have to scroll around the screen more.
Open a web browser. Clear the URL (the address in the bar at the top of the screen) & type in 192.168.1.1 then press 'Enter'
(This will not work if you are in the search bar and not in the top address bar)
Password is - admin
Look at your connection status on the home page. If it is on, turn it off.
Click 'Network Settings' or 'Network Information'
Click 'APN'
Click 'Manual'
Click 'Add New'
Click inside the box where it says Profile Name
Type in 'tmob' (do not put in the apostrophes)
Click inside the box next to APN
Type in 'b2b.tmobile.com' (all lowercase - do not put in the apostrophes)
Click 'Apply' (it should say success)
Click 'Set as Default' (it should say success)
Unplug the modem from power or power the modem off with the red button on the back.
Wait 10 seconds.
Plug the modem in again or power it back on using the power button on the back.
WAIT 60 SECONDS. It is very important that you don't try to connect to the modem for 60 full seconds. If you have devices that automatically connect to it, that's fine. Just don't use them for 60 seconds or try to log into the modem.
If it is still not working, log into it again. If the connection status is still set to off, then change it to on.
You can also try the alternate APN.
Profile Name - tmos
APN - b2b.static