iPhone 6S No Service after iOS 12 Update Issue

After downloading the iOS 12 update on my iPhone 6S (which was very tedious, might I add), it started to act very strangely. For the past five days or so, I have been getting a "No Service" icon instead of my usual T-Mobile carrier logo. I kept repeatedly resetting my phone in order to get my data back and it had worked for a while, until I downloaded the iOS 12.0.1 update. Ever since, I had no way to connect to the Internet besides using any available wifi.

Resetting the network settings did not work and I had recently erased everything on my phone, but I still have no service. Now my phone is stuck at the manual setup section, claiming it is "Unable to Activate". It says that "an update is required to activate your iPhone".

Can somebody please help me solve this issue? I am literally unable to leave the manual set up screen now without any service so I need someone give me some useful advise right now. Thank you.