iPhone 7 Model A1660 has "no service" issues even when my cellular coverage is available

My iPhone 7 Model A1660 has "no service" issues even when my cellular coverage is available. The apple program covers affected iPhone 7 devices for 4 years after the first retail sale of the unit. I bought it less than 5 years ago and it was very expensive! How can I get it fixed even when it had been bought more than 4 years ago?