Patience is the key.
Bring the timer near the phone. Take out the battery, wait at least an hour. Put battery back in, wait about half an hour. Give everything time to settle.
Then make sure the device has been removed from the app, and start the ‘add device,’ process again.
You must be connected with Bluetooth and very close to the timer for a firmware update. If successful reconnecting, stay on bluetooth and you can try the update again, after ten minutes or so. (I have rescued a couple of timers with failed updates.)
Very curious how you were able to attempt to add device from web portal. Please explain.