iPhone 5 “no cell coverage”

I finally factory reset my iPhone 5 around a week ago after having it or using it for many years, but even when I’m not using it, the battery drains from 100 to 3 percent overnight. And I check the battery settings and of course, there’s no battery health setting as far as I can see because it’s on IOS 10. But all of the other questions on this page is asking about iOS 15+ and I’m not sure how to solve it, I truly hope I don’t have to replace the battery because I don’t have enough money to do that.