Vivosmart 4 reading MISSING

I have just paired a new Vivosmart 4 for the first time, successfully; but while exploring menus the Vivosmart screen went to MISSING.  

I've tried to reconnect by deleting the Bluetooth pairing in my connected device, an iPad Air running iOS 12.4.6 and pairing again; and I've had the Vivosmart 4 in a charger for over 2 hours.

But the MISSING readout persists, and I cannot pair.  Think the Vivosmart needs to be reset, but removing from charger then reconnecting fails -- and the reset procedure itself is a menu item, which of course I can't get to.

Help very much appreciated!