I understand you're having some trouble connecting to your mobile network. I want to help you get your iPhone connected to your network again.
First, let's make sure you're running the latest version of iOS 11.3.1, you can follow the steps here to backup and update:
Update your iPhone, iPad, or iPod touch - Apple Support
If your iPhone is up-to-date, do you see any error from your mobile network? Are you seeing a message that says Searching or No Service in the top left?
Follow the steps here if yes:
If you see No Service or Searching on your iPhone or iPad - Apple Support
Let me know if that helps, or tell me more about what's going on and I'll help out.
Thanks for using the Apple Support Communities.