Hello,
Welcome to the Microsoft Community.
Surface Pro 3 is supported with 5G signal.
Please try to change the 5G signal channel on your T-Mobiles router (you can try to change it to channel 30 or 40).
Also, if possible, please try to connect to another network with 5G signal to see if you can connect successfully.
Best regards,
Huy-K | Microsoft Community Support Specialist