Hi,
After resetting TP-Link, did you make sure it’s in client mode? In client mode, it shouldn’t need to request DHCP itself because it would simply pass packets back and forth between OpenSprinkler and router.
In order to isolate the problem, my suggestion is to connect OpenSprinkler to your router via a Ethernet cable. You can place OpenSprinkler close to your router so a short cable should be sufficient. This is just for testing and not a permanent setup. See if the problem persists. If it does, that probably means the issue has nothing to do with the TP-Link adapter.