The GoPro Range Extender was originally build for the GoPro Hero 3 Black Edition (WIFI enabled) but should work with any WIFI enabled device.
Enclosed are screenshots from the DD-WRT configuration page.
GPH3BETTL = The GoPro Hero 3 camera
IP = 10.5.5.9
GPRepeater = the TP-Link router
IP = 10.5.5.10
First make sure you can use your router as a normal repeater by using it as an repeater for your normal WIFI. Then change IP adress to 10.5.5.10 and reboot router. Now you can't connect to it. Manually set your computers IP adress to 10.5.5.100 and connect to the router for further configuration.
Same goes for your smartphone. Set the connection info for the GP-Repeater connetion to IP 10.5.5.10x (10.5.5.100) to access camera/router.