How to fix No Service on my iPhone 7?

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 Bar 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.

Genius Bar Reservation and Apple Support Options - Apple Genius Bar Reservation and Apple Support Options - Apple

Temporarily, use another phone to contact your carrier, so you can get cellular service restored on your iPhone again. The carrier may need to send you a new SIM card.