Do you mean the 5G wifi will disappear when you download/upload? Or it just disconnect and need to connect again?
How far is the device from the router? And how many devices are connected to the router?
Kindly upgrade to the latest firmware version, can try different channels for 5GHz, and change the channel width to 20MHz and reboot, then see if it works better.

Enson Loo wrote

Do you mean the 5G wifi will disappear when you download/upload? Or it just disconnect and need to connect again?
How far is the device from the router? And how many devices are connected to the router?
Kindly upgrade to the latest firmware version, can try different channels for 5GHz, and change the channel width to 20MHz and reboot, then see if it works better.

Thank you for your suggestion!

If i change the channel width to 20MHz, it is much stable than before. But the upload and download speed was reduced significantly.

When I choose channel width as Auto mode, the Wi-Fi is going to disconnect and re-connect during large volume upload and download.

5.0Ghz Wi-Fi can be only use by one user only for playing app games. Other users shall use 2.4Ghz Wi-Fi

My C2300 also runs into troubles with 5GHz bands.

The router reboots everytime when I try to connect to PPTP VPN server on remote sites from a Windows 8.1 PC. This does not occur when I try the PPTP VPN connection over 2.4GHz bands. In addtion, PPTP VPN connections from Android over 5GHz bands also have no issue.

This is inconvenient for people who needs to work from home with connection to remote site through PPTP VPN.

My router is an Archer C2300 v1.0 running firmware version 2.0.0 Build 20170922 Rel. 32364.

This router worked well as far as I could tell for ~24 hours. Now today, wifi is a mess over both 2.4 and 5 ghz bands. 2.4 suddenly very unstable and intermittently getting 50 Mbps tops internal/external. 200 mbps over 2.4 the night before. Can't even connect to the 5 ghz network anymore.

Firmware is updated. Even turned off encryption. Still get error that the network could not be joined. Distance to router is 1 foot. Ethernet is unaffected. Reset router to factory and reconfigured. No effect.

Through trial and error I narrowed down the issue to the Multi User MIMO Feature at the bottom of the 5 ghz advanced settings page. Disabled it and now I can connect to the 5 ghz and significantly improved wifi speed.

Anyone else care to try this and report results?

Same here, i have an Archer C2300 v1.0 running 2.0.0 Build 20170922 Rel. 32364, it takes a life time to 5.0 ghz signal stabilize, i already tried to factory reset and nothing....

But one thing i realize, when i use a high value for channel ex: 149 to 161, i get a more stable signal even after a reset, for me the issue is more frequently when i use low channel values ex: 36 to 48.

Don't know if is a firmware issue or hardware issue....

And, Ephedup, could you try to reset after disable MIMO?

I tried your test but without success, after a reset router the issue persists.