Not an ideal solution but if it won’t go into a true bridge mode, you could just DMZ the OpenSprinkler’s IP and set a static IP on the WAN interface (the wireless side).
Of course, it would be better to find an option to truly disable routing but hopefully that at least gets some networking up for you.
Edit: This guy seems to have it working in bridge mode: http://www.hongfire.com/forum/showthread.php/411367-SoftwareGuy-s-Review-LB-Link-Chinese-Pocket-802-11n-WiFi-Router. Might help.