Im having the same issue.
After no obvious tech support above. Im very surprised Shelly havent addressed this critical issue
So I found a fix...
1/ use a computer or mac.
join the wifi newtork of the shelly device you want to add.
2/ go to your browser and type http://192.168.33.1/
3/ refresh and wait. I may not join straight away.
4/ Goto internet > Client mode > input your wifi details there ..
5/ Then goto the shelly App on android or Iphone and refresh by dragging down the window, until you get discovered devices. press that
6/ Now look at the IP adress on the mobile screen!
7/ Go back to your PC and type in the Address in a browser
8/ when it loads, goto firmware update and wait 5 mins for it to update.
9/ Now back to the app and you can add.
repeat this for every shelly 2.5 until shelly fixes this