@whovian 

The WiFi standards have changed over the years and now 40mhz widths are common place on the 2.4ghz frequency, this  totally negates the old rule of channels 1, 6 and 11 which are based on 20mhz widths. 

Add into the mix that 2.4 is pretty much flooded in near all situations, there is no guarantee that 1, 6 or 11 would be the best for you.

It sounds like the Deco has scanned the airspace and selected 8 as the best for you, that again is likely 40mhz so will cover from around channel 3 to 12 in terms of frequency.  40mhz widths only allow for 2 non overlapping ranges.   

Remember that overlapping isnt an issue, the only time you need to be concerned with it is if the dbi interference is strong enough to cause issue.   Say for example 3 APs in the same lecture theatre, or sports hall.   If your neighbour is on the same channel as you, but showing -75db then its such dead air it isnt going to cause you issues on that channel.  With 2.4 there is going to be interference no matter what channel you select.

Legacy devices have to support channel 1-11 so its highly unlikely to be channel selection causing the issue for you.   From 15+ years of experience, 2.4ghz is flooded by todays standards and you are never going to win that battle.   Performance on 2.4 is an issue for everyone sadly!