Earlier today my sister needed to used her iPad Pro 9.7 to connect to the internet via my cellular data. So I popped out my sim card (T-Mobile) from my Nexus 6p and insert it in her iPad Pro. The iPad then asked me to activate the sim card to the iPad by entering the phone number and SSN# associated with the sim card. I entered the info requested and my sisters iPad was able to connect successfully to my cellular network. After she finished with what she was doing I put my sim card back into my Nexus 6p and that's when I noticed that the phone was not recognizing the sim card anymore. It is now displaying "Emergency calls only". I have tried to fix the APN settings for T-Mobile. The Nexus 6p is Carrier Unlocked by the way.
Screenshot of my phone:
https://postimg.cc/image/v97y194wt/
Screenshot of my phone:
https://postimg.cc/image/v97y194wt/