Though Rachio is not good about reconnecting after an outage, it very rarely ‘forgets’ the stored SSID (network name) and password. Just remove AC power (at the wall, or with the connector from the power adapter), wait 10 seconds, then plug it back in. With luck it will reconnect, without using the app at all.
However, if you messed with the controller buttons, you may have reset the stored credentials and must reconfigure them. Follow just step 2 in
and with luck the controller will get to the ‘Awaiting Wi-Fi’ state. See
Then, use the Settings app on your phone to reconnect. See the first post in
If you can get the controller to broadcast its Rachio-xxxxxx SSID but still have trouble, see
If still no luck, report what you have tried and observed, what light states you have been able to achieve, and what devices you have that may be useful (old phones with no SIM, tablets, laptop computers).