Reboot your XT8. Select 160mhz if you can and apply.
If you can't select 160mhz or you can't manually pick a channel then DFS is either waiting or has denied a channel. This can bump you down to 2.4ghz.
Go to System log > Wireless log and look for the following
If you see the "Temporarily Out of Service for XX minutes" then DFS is not letting you use that channel.
I am in the following scenario.
For those in the UK. Channel 124 (5620mhz) is the same a the Met Office Weather Radar and is smack bang in the wrong place to allow 8 contiguous channels for 160mhz on this Mesh system.
If you live near any of the Radars in the attached map you are not going to be getting 160mhz bandwidth.
View attachment 25366
Here is what can be done to fix the issue.
1. Asus give us 80+80 allowing us to skip this channel.
2. Asus open the 36-64 channels allowing us more choice on where to stick our 160mhz and can stick front end on 4 contiguous channels in the 100-140 range.
(This would work for me fine as 36-64 are not blocked for me and frustrating as 36-64 and 100-140 are available for the single 5ghz interface on my RT-N66U).
3. Place the router to the side of the property towards the nearby radar facing back into the property and put a tin foil shield between the router and the nearby Radar. I managed to get this to work on one router but not the node unfortunately. The idea being that it stops the Radar frequency hitting the router and DFS blocking the channel.