I recently changed routers and had a very challenging time to get WIFI working on my Gen2. Original routher was 2.4G only. Everything was seamless.
I found some very simple solutions to the problems which I spent many hours working on that I hope to help others.
First off as online support will tell you, the device only works on the 2.4G network. My router has both 2.4 and 5 G.
With the app on and trying to get on Wifi, I can see the router name for 5 G flash on the screen and then move onto to other router names. When it gets to the 2.4G Wifi, it refuses to work there either with the correct password entered.
Here is what I did to make it work. On your Iphone or Android, you can not be on the 5G Wifi network and sync with Rachio. Your phone has to be on the 2.4G Wifi network. The second I did that and repeated the steps, Rachio Gen 2 automatically takes the password stored in your Iphone and completes the Wifi integration. Nowhere could I find help that pointed to the fact that Gen2 and your phone need to be on the same Wifi network. A 5G Iphone will not work with 2.4G Rachio Gen 2.
Hope this helps someone as I struggled to find this out the hard way.