Orbi RBK852 - Selection of 5GHz channel 36

What Firmware version is currently loaded?
What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too?

What browser are you using? Try a different browser? 

Be sure to clear out all browser caches before entering the RBRs web page. 

Probably it's a bug or a problem with the firmware, would pass all the data in the case you created so they can examine it.

First try with the previous firmware version and check if the problem still occurs , 

As it is likely to be a firmware issue, try to return to the previous Firmware ,  and or upgrade back to newest one .

Also check the realease notes.

Setup: Calix ONT > Orbi RBK850 (FW: Latest)> LAN/WLAN (2 - RBS850s)

Although I wish NG would open up the number of channels we can use on the 5GHz end. Everybody in my area equipment, although there's only one that is IT enough to control his channels. The rest would just let the routers do their thing and all seem to follow me around and crowd the channel I'm on.

I found that visually when set to 36 it looks to be centered on 42. The app I use Analiti shows both what is being seen and statistically and the dominant frequency is 40.