So this is strange. At home I have 2 WiFi networks ‘Home’ and ‘Home_Guest’. My HA was setup on the ‘Home_Guest’ network.
On my iPad when at home I can login into HA regardless regardless of which WiFi network I am connected to. On my phone if I am connected to the ‘Home’ WiFi it will not connect, I have to switch to the ‘Home_Guest’ WiFi network. Once logged in I can switch back to the ‘Home’ network and it works fine. The problem only occurs after I am out of WiFi range and come back in.
Click on your server name at the top of the window.
Scroll down to internal URL and tap it.
Make sure both wifi SSIDs are added.
Now there may be one further complication. Most guest networks enable client isolation. So one wifi device cannot contact another. If that is the case with your guest network it would be best to move your phone app to the Home network.
Many thanks, I think maybe the 2nd part of your answer may be pertinent. When I add the guest network it says ‘error saving url’. So I saved it anyway but it made no odds, I will move HA to the home wifi.