As per our official statement on cellular carriers that work, only T-Mobile & it's MVNOs (ex. US Mobile, Tello, Mint Mobile, Metro by T-Mobile, etc.) works with this phone.
If you are using a different carrier, it is not expected to work.
If you are using T-Mobile or one of it's MVNOs and it is not working, please follow the below troubleshooting steps.
Reset APNs
On the device, go to Settings -> Network & Internet -> Mobile Network -> Access Point Names (APNs)
On the bottom left press the 3 lines ( ≡ )
Press Reset to Default
Restart the phone and see if it works.
Request a new SIM Card
Sometimes when you are using an already used SIM card, there are lots of configurations on it and data that you aren't aware about that cna be affecting the service. Request a new SIM Card from the Cellular Carrier and see if that fixes the issue.
Ensure it's T-Mobile
Make sure that your cellular carrier uses T-Mobile as their network. As stated above, only T-Mobile & it's MVNOs work with the phone. Contact the Cellular Carrier to see if they use T-Mobile as their network.
You may have to switch carriers if they do not use T-Mobile as their network.
Call the Carrier
Sometimes when you switch SIM cards, the carrier locks it in case it was stolen. Call them and see if they placed any restrictions on it.