What is this

I have a message come up that tells me I cannot make or receive calls nor can I send or receive txt messages , reads that my cellular needs to be updated. What is this? I have never gotten anything like this before I have been getting them for 2 weeks off and on. I turn off my phone and restart it and at times it works fine. But it came up again tonight. I don’t have an Apple store near me nor do I have a computer it says to fix this

Not in messages just comes as a banner across my screen