CELLULAR UPDATE FAILED- No solutions working

Yesterday night I was on a group call with a few friends on whats app and suddenly out of the blue my phone restarts. I thought the battery died but when I turned it out, it was at 66%, which was odd. A few moments later a message popped up on my phone saying, Cellular Update Failed. I have an iPhone 7, A1660 model, and bought it roughly 2 and a half years ago, facing no issues. At the time I did not think much of it, and two hour after I was starting to get worried. I google the issue and found a ton of articles on the issue and tried all the solutions, even the official apple one with connecting your phone to iTunes.

No solution has worked and now its getting frustrating. Its really inconvinient to have a phone that cant be a phone. I took out my sim and put it in an old iPhone 4, it worked. I put it in a old Samsung galaxy, it worked. I live in a country with no official Apple stores or Genius bars which is also frustrating. I am also a student, so purchasing a new phone isn't really a small expense.

I am now using a old iPhone 4 to make calls, but I cant really do anything else with it since its old and lags alot. Please help me with any fix ASAP, thank you.