How come my iMessage number changed

How come my iMessage phone number changed to a TextNow number I have from an that app. I’ve gone into settings and everything but for some reason under settings, phone, and then when I click on the phone number (which isn’t the number I want. It’s just a new texting number that for some reason has hijacked my whole phone) but when I click on it and delete and then type in the correct phone number, I hit save but it doesn’t save. It keeps the texting app phone number and not my usual one.

on another note, the phone number listed in settings and in the TextNow app is the correct number but it’s not using it.