XS MAX: Unable to Activate, Update required.

After my iPhone XS Max updated its iOS (the last update before iOS version 13.7), I encountered an error for cellular data update failed. I brought my phone to apple service center and after they "restore" my phone to the last working iOS version, my phone now is unable to activate. They said they also tried updating my phone to the latest iOS version but still my phone is unable to activate.

I also tried connecting my phone to my computer using iTunes but it cannot detect my iPhone. I turned off my iPhone, removed/disconnected from the computer, closed the iTunes, turned on my phone, connect to my computer then open iTunes and it now says unable to activate because the activation server is temporarily unavailable. Try connecting your iPhone to a Mac or to a PC with iTunes to activate it, or try again in a couple of minutes.

Anybody who also experienced the same problem (UNABLE TO ACTIVATE, UPDATE REQUIRED)? Any solution that you can recommend to have the phone activated again? Would appreciate any help/information that you can share with me. Thank you!