How to upgrade an iPhone 7

First, try contacting your carrier to see if they can help you with their service. If not -

There was an inherent issue with some of the iPhone 7's. The hardware would fail leaving the phone without cell service. Apple acknowledged this and repaired it for free for many years. However, that warranty repair expired a couple of years back.

If your iPhone 7 no longer works with cellular, you will have to purchase a newer phone.

You might be able to take it to the Genius Bare to have it repaired, but I'm not sure if they still have the parts to fix it and if they did, it probably wouldn't be worth the price of repair.