Cellular Data on my iPhone 12 is now "off" and I can't turn it back on.

Cellular Data on my iPhone 12 is now "off" and I can't turn it back on. I have tried powering the phone off and on. But it remains "off". What should I do? I should add that I was just starting the process of adding an internet service to my home, but I didn't do anything to shut off my desperately needed cellular service on my phone.