Could you find out the IP address of your phone or laptop when it’s connected to the same network? I am curious to see if they have the same IP address or maybe your guest network is assigning IP in different address ranges.
I don’t know what’s the best way to proceed. If you need to program the controller, you can keep it in AP mode — that way itself behaves like a router. You can have your phone connected to that AP. As long as you use the OpenSprinkler mobile app, you can change settings and set programs (in AP mode the controller has a fixed IP of 192.168.4.1). That way at least you can start using it.