Dropouts - help please

[K4680]

I'm getting occasional dropouts. 

System is all wired:

Sky Q modem -> Ethernet -> Airport Extreme -> Ethernet -> Expert Pro 250

I'm using Connect HD from iPhone or iPad and sometimes bubble uPnP from Amazon Fire HD.

I was using an ASUS RT-86U as the access point/switch, but swapped in an old Extreme that was sitting unused for years.

Both operating on a 5khz network.

I think the problem may be that the ASUS / AE  were sitting in a cupboard (all my hifi is in an enclosed unit out of sight). 

I have moved the AE outside and so far no problem. 

Is there a line of sight issue with 5Ghz, or anything else that might be the reason?

Difference between the 5 GHz and the 2.4 GHz frequency band!     
 
Interference:
A 5GHz network has a smaller problem of taking up interference because most wireless devices such as Bluetooth devices, cordless phones, microwaves, and computers use the 2.4GHz frequency.

Reach:
The 5 GHz band has a shorter range compared to a 2.4 GHz band, because in radio frequencies, the higher the frequency, the shorter its range. In other words, if you use a lower frequency such as 2.4 GHz, the distance that will be covered will be greater than with the 5 GHz band.