iPHone 5 supports only iOS 10.3.4 which was released on July 22, 2019 - TextNow only supports iOS 11.0 or later
There was nothing you could have done. You can't use the iphone 5. The 5S is cheap. you can get some for under $50 on ebay. Or get a 6 for close to $100. Which is a steal because it looks like an iphone 8 but costs 1/5th as much.