@mynameis 

Try this: enable the Guest network but for 5GHz only.  Then try to connect the Apple device to it.  If that does not work please let us know more detail on the Macbook.  OS, hardware version that sort of thing.  We would need to provide that to our engineering team to further look into it.  

@Carl I implemented your suggestion (5 GHz only Guest network) yesterday morning and the MacBook Air has successfully been joining the Deco Guest Network for the past 1.5 days.  I think it has connected every time without problems.  What do you think this means?  Ideally, I don't want to keep a Guest network and my main network going.  I'd prefer to only have one main network SSID.

Also, when the Guest network is active, is it competing for air time with the primary 2.4/5 GHz network, or is it using the backhaul band instead?  I'd like to understand what impact the Guest network has on my network performance.

@mynameis 

My guess based on expeiance is that its an apple issue.  Apple products tend to like to connect to and do their best on higher channels which could be leading to the connection issues.  Another thing it could be is becasue both 2.4GHz and 5GHz are broadcasting on the same band there is a chance the adapter in the computer is confused by which one to connect to.  We see this alot with Range Extenders and apple products when the extender has the same name as the router.  The device is confused by which network AP to connect to so it doesn't connect to either.  I don't know if you have the ability to do this but if you can set your Mac's adapter to AC only that would eliminate it from even seening the 2.4GHz band and might clear the issue up entirely. 

Hi @mynameis .

you might want to look into Wifi country codes as reported in other threads here with MacBook issues : Tp-link decos might have fixed CountryCodes that do not match your country. 

Other members here suppose this might cause the Macbook troubles (given that authorized wifi channels depend on countries)