Definitely try to reboot. Also make sure your sim wasn’t dislodged in the sim tray and wasn’t upside down. The tray is keyed but you’d be surprised how many people will force the sim in or that it gets dislodged when being inserted.
Allow 24hours for it to kick in. Double check your network is on T-Mobile. You may have to take the network selection off automatic in order to select T-Mobile.
