I am running into the same problem only with my iPhone 6s. I bought an iPhone 8 and want to use my old phone as an iPod. It should be working only it isn’t. My iPad works, my Mac...
I can send regular SMS text messages from my iPad and my Mac when I am near my iPhone 8 to people who don’t have iPhones. I did the same thing when I was using my old phone as a phone. Is that what you are trying to do?
I talked to Apple support today and they will be calling me Tuesday afternoon to follow up. If I learn anything new, I will post it here. And if anybody learns anything new, I hope you will post as well.