How long ago was this - things were slightly different in the days Sky had powerline, although it was never recommended an engineer leveraged these (they often did though) which caused a bunch of issues when Sky went onto disable powerline for everyone.
The mesh network (network between the extra boxes) will still use 5GHz over its own wireless network. The main box, will only communicate to the main router over 2.4GHz when sky q routers are not used. All other boxes will still be 5GHz over a link from main tv box to sky q minis. So it would go non sky Q router - main box (2.4GHz only) - mini boxes over 5GHz (using the mesh setup on the main Q TV box).
For that initial link to also be 5GHz it must be setup over WPS on sky hardware.
This is commonly documented, and I have in fact used my own hardware. If this is incorrect I would need to see how you managed this. 5GHz between the sky main tv box and Q mesh, is only activated when using WPS setup on the Q router and Q main TV box. Third party routers require you to use the second option when configuring.
Sadly I am unable to replicate this as I no longer have Sky Q as I moved to an area with Virgin Media and it was cheaper and they could install sooner.
My network only ever has a 5GHz SSID visible the 2.4GHz SSID is hidden and only used for smart home devices, also I didn't have Sky broadband either
When the Sky engineer installed it I only provided him with the 5Ghz network and he set it up and did the WPS setup with the Mini boxes.
Some users have managed to turn it back on, but not recommended as there's no support for it.
As it stands, as the 5GHz wifi is the only thing the mesh can use now, there's no way to add a third party router to that, as 5GHz is kept "clean" for the mesh comms, and 2.4GHz is "free" for non-mesh products. Back in the day, powerline could be a backup, but again, was never a recommended approach. Controls were less tight back then.
Edited by ukhardy07 (Fri 04-Dec-20 11:19:19)