I am a bit confused by this part:
“Secondary Router set to “AP Mode” in garage – NetgearWNDR4500”
I assume your OpenSprinkler is connected to this secondary router. If it’s in AP mode, then that means any device connected to it is on a different local network than your main router. So you will need to set up port forwarding on your secondary router (NetgearWNDR4500) in order for any device connected to the main router to reach OpenSprinkler. On the other hand, if your secondary router is a switch, then you probably don’t need to do so.