I have updated the ios 13.3 update, but my phone is showing NO SERVICE...cellular service failure....how can I fix this?

I have an iphone 7. I have gotten the ios 13.3 update, but I'm not sure it's installed to my phone. I have NO SERVICE, and it says I have a Cellular Failure. I have turned the phone off and on several times. Sometimes I do get service, but then the phone will go black and when it comes back on I have no service. Mostly I have no service. I can text some people but not others.