I can see only the 2.4 ghz and 5ghz-2 channel on devices.

I bought this modem around Sept 2019

@Kevin_Z Hi, i have the same problem. and now the internet drop. I think that new firmware on the problem (especially on QoS feature). I compare to tenda router and its fine, the internet speed is 43 Mbps.. And on the tplink just get 35Mbps and not stable. I've tested on 3meters distance on both router (tenda and tplink). Please help.

@ardik 

I am having the same issue with a refurbished c4000. They sent me a refurbed model to replace my a20 because I couldn't get strong signal on any network unless I was 2ft away from the antenna. The a20 worked flawlessly for 14 months before it started to fail.

I am having an issue with my Archer C4000 that seems related. I'm using the smart connect, and have been for about a year. I noticed that when moving to one part of my house that the WiFi goes out and comes back sporadically as I try to reset my connecting device's wireless. I've discovered that the common condition to different devices experiencing this is that the device is connected to the 5ghz-1 radio. Once I get the device to use the 5ghz-2 radio there is no issue. 
I've since monitored the router more frequently and see that all devices use the 2.4ghz or 5ghz-2 radios all the time. 
So it seems my 5ghz-1 radio has an issue. I'm planning to stop using Smart Connect, which is unfortunate because it's a nice feature for a family of users, and go to a setup that doesn't use the 5ghz-1 radio. Is this a hardware or software issue?

I'm mainly posting so TPLink and others can see a history of a potential issue.