If I want to extend Dumb AP (channel 40) , the repeater must be on channel 40? Or should I pick another channel?
depends on the repeater, does it have one, or two radios ?
Wifi 2.4 and wifi 5
Im talking about using 5ghz as repeater for example..
if you use the same radio for receiving and retransmitting, some (most ? all?) won't let you select a different channel for the retransmit.
If the "repeater" is using wireless backhaul such that it is wifi-wifi (and no wired connectivity), what @frollic has said is correct -- unless you have multiple radios (in the same band), the channel must be the same.
If you are wiring this with ethernet (so that it is a dumb AP instead of a 'repeater'), you should use a different channel than the other AP(s) in your space.