On the bottom of page 77 of the manual it says:
"If the OBi is set to function as a LAN switch rather than a router, OBiWiFi is disabled internally."
So it looks like you have to use the router mode, not the bridge/switch mode, if you also want to use WiFi. This sort of makes sense, since the wired side and the WiFi side might be on very different networks, and you couldn't/shouldn't/wouldn't want to bridge them, you need to route between them. The OBi doesn't know that it is just a printer your a connecting, and doesn't realize that the wired LAN doesn't have its own Internet access.