2.4g connection unstable while 5g works perfectly

I kind of worked it out what is going on.

I found that the only thing I need to do is to disable the "20/40 MHz Coexistence" and everything just came back to normal. 

I don't know how to put up a picture here, but anyway I tried to use my Dropbox link as below to show you.

Disable 20/40 MHz Coexistence

I think I did the 2.4Ghz channels right by reset the channels with each router/AP, and I'm quite sure that the signal/channel should not have much overlap or interference in between, but after a few hours of trying I still had no luck. 

When I tried to go through all the settings after reset the router to factory default, I found that originally when it is default to be 20/40 MHz. Some articles in other post showing that the system will most likely to use 40 MHz as preferred setting. Then I went back to see all the other router/AP's setting, found that all of my other routers/APs set the frequency width to be 40MHz. I guess that should also means better performance at some level, but unfortunately in my case the wider frequency width from the other routers might just squeeze out the available channels for my R6900.

Anyhow, after disabled the 20/40 MHz Coexistence, the problem seemed to be gone for now. I will stick around and do some more tests and if I got any I will post it here then. 

I guess my issue is not very common in most cases, but hopefully my experience can provide some help to those who needed!!!