A repeater must be on the same channel as a gateway within range to mesh properly. A repeater should have at least -65 dBm RSSI for best results.
While we recommend using the Auto Channel feature to ensure gateway APs use on the best channel, networks with many repeaters may benefit from manual channel assignment to ensure the repeaters use the best gateway APs.
First make sure that the channel setting under Configure-> Radio is set to manual. Then under Manage->Access Points click the AP and and select the channel in the Radio box.
For more information about channel use, see: Which channels should I use?