Poor 5 GHz signal with Draytek 2860N router

[Quartz]

I've got a nice new Draytek 2860N router and it's working fine except for 5 GHz wifi. In the same room it's fine but as soon as I go to any other, the signal drops drastically. In my office, through 1 brick wall and a frame, there's barely a signal on the 5 GHz band, but the 2.4 GHz signal is excellent. I've tried changing the channels without improving the situation. I'm in a block of flats, with flats above, below, and to one side.

Any ideas?

 

[SC0TTRS]

Higher frequency = shorter range, 2.4Ghz has better penetration when it comes to solid objects over 5Ghz. If the Draytek was capable of 802.11ac it would improve this issue but it's limited to 802.11n.

 

[Caged]

That's what 5GHz does.