My wifi socket only connect to 2.4ghz, it fails all time with RBR20

[kwanmomo]

Hi all, I am from Hong Kong and I have bought a RBR20 two weeks ago, it woks fine with most of the devices when I set the username and password as same as my old router. But for my CCTV and also wifi sockets, they started disconnecting irregularly. And this week one of them disconnected again suddenly and I try to reconnect them but it just failed. I try to set in the advanced mode in the computer to disconnect the 5ghz (untick the two boxes) and see if the 2.4ghz works to connect my device, but it finally failed. I am very frustrated can someone help me, thanks. 

[FURRYe38]

What FW is currently loaded?
What is the Mfr and model# of the IP modem the NG router is connected too?
What is the Mfr and model# of the ethernet switch if one is in the configuration.
What is the size of your home? Sq Ft?
What is the distance between the router and satellite(s)? 30 feet is recommended in between them to begin with depending upon building materials.

What channels are you using? Auto? Try setting manual channel 1, 6 or 11 on 2.4Ghz and any unused channel on 5Ghz.
Any Wifi Neighbors near by? If so, how many?

Try disabling the following and see:
MIMO, Daisy Chain, Fast Roaming, IPv6 and Set 20/40Mhz Coexistence to 40Mhz only. Save settings and reboot the router and satellite(s).

Not sure if this could be of any help:

Advanced Wireless Settings both 2.4GHz and 5GHz changed CTS/RTS Threshold to 2307.
https://community.netgear.com/t5/Orbi/Orbi-RBR40-RBS40-V2-1-4-16-mesh-network-drops-connection-to/m-...
I changed the CTR/RTS threshold values for both 2.4 GHz and 5 GHz (2341 and 2345 respectively), changed preamble modes to long preamble, and Set 20/40Mhz Coexistence to 40Mhz only.