It’s 1x 2.4GHz and 1x 5GHz channels. The 2nd 5GHz is used for wireless backhaul between the Deco units so it’s a private channel. It’s not really used with wired backhaul but if the 5GHz channel used by devices gets over loaded it can take some of the private backhaul channel and help it.
The Guest SSID is off by default.
The Main SSID and Guest SSID have toggles for both the 2.4GHz and 5GHz channels. You can select any combination you want for each.
Wifi extenders and repeaters bring up the old adage of “Garbage in Garbage out”. Most people will place the extenders close to where they are not where the signal is good to boost. You also lose MIMO going from your main wifi router to an extender/booster.
With the M9 Plus and its dedicated wireless backhaul channel it sort of solves this problem. You still need to place the units in a good spot.