Under settings there is network, have you altered that in any way?
i take it you have rebooted the iPhone as well.
Have you tried to reinstall the app?
If HomeKit works off the same phone it maybe the app that’s not playing fair. I’ve had a few issues with their app, it was impossible to update until recently on my iPad Pro yet an old iPad mini on iOS 9 could. Same with out of home control, that was really unreliable for me and again a recent update has solved it.
If all else fails You might have to go a complete reinstall and start fresh but I do wonder if the app is to blame.