It’s possible the TP Link is acting as the DNS server for the OpenSprinkler and not replying correctly. I am just suggesting a possible source since the issue is only behind the TP Link wireless device and otherwise works in your wired environment.
If you could, explore the DNS settings on the TP Link and verify they are using your router’s DNS or just put Google’s DNS:
8.8.8.8
8.8.4.4
Otherwise, I am not sure why it would work when wired and not on wireless but still allow you to control the device. I say this because being able to use the app or web interface of the OpenSprinkler proves IP connectivity is working and the only missing piece is DNS which is only used for the weather DNS lookup.