range extender lose connectoin every 15 minutes

This happens also to me with the netgear WN3000RP but not so often as you mentioned, i have it with the speedtouch 780wl but it seems that sometimes the dhcp adressess a new ipadres to the laptop or other device connected through the extender wich all get there ip from the thomson dhcp but these routers don't have enough options to change the dhcp for longer times. only thing you can do is set the ip from dynamic to static. In the wireless option you might want to turn off multicast setting try that for better coverage of the signal.
try the program inssider to check if there are more routers with the same channel you are on!