Having bought my OpenSprinkler I first tried to connect it via WiFi by buying the successor of the recommended WiFi Router, a TP-LINK TL-WR802N. As the controller is located under the surface about 20m away from our house it didn’t get a connection.
So I thought about PowerLine and bought an AVM FRITZ!Powerline 510E Set which connects LAN ports on both sides via the power cord. Again the signal was just a little bit too weak to work. I placed one of the two PowerLine Adapters in one of the sockets nearest to the socket connected to the OpenSprinkler and the signal was good. So I bought an additional AVM FRITZ!Powerline 546E adapter which also features WiFi. I expected to set up a WiFi bridge between my router and the in-house PowerLine adapter and wanted to have the connection extended via PowerLine to the OpenSprinkler. Unfortunately AVM didn’t thought about that use case and disables the PowerLine connection when the device is switched to WiFi Bridge mode.
In the end I attached the TP-LINK router in client mode to the 546E PowerLine and set the later to PowerLine bridge mode. The connection is extended via PowerLine to my OpenSprinkler in the garden which is connected to a 510E PowerLine adapter. Weird setup but it works. I suggested this extended WiFi bridge mode to AVM but don’t expect them to implement it. But hey, you never know.
Just wanted to share this use case as there might be someone out there with similar problems connecting his OpenSprinkler.