While connected to your routers wifi network, login to the admin control panel. The default address is: 192.168.100.1, and the default password is: admin

From the home screen, select the APN option from the menu under 5G NetWork 

Select The Add APN button

Fill in the APN field: fast.t-mobile.com

Click Add

Select the radio button for the newly added APN

Click the Save button

Restart your router then verify the setting has taken affect.