Homekit remote access stops working when iPhone leaves the premises

iPod Mini as a Homekit hub.

Until recently, all work working well.

Now I find that when I leave the house, with my iPhone, the Homekit hub reports no connection and Homekit is not remotely accessible.

Simply adding the iPod in the Home app by holding the iPhone near the iPod fixes it instantly

This problem seems to occur when the iPhone arrives at or leaves the house with WiFi on.