@GalvinNguyen 

How about the speed on the 5GHz?

WIFI has two Bands - 2.4 GHz and 5 GHz. 
The 2.4 GHz is slower but works at farther distances. 
The 5 GHz is faster but not does work as well at distance. 
The SSIDs should be different for best performance and enables you to put specific devices on a specific band. For example, you may want to put printers on the slower band and your PC on the faster band. 
 

Other devices may be interfering with the radio signal. Things like cordless phones, wireless security cameras, anything wireless that uses the 2.4 or 5 bands. 

Kindly set the channel as Auto, it will detect the interference and choose automatically.

@Steven.Zhang 

I separated 2 SSID for 2 channels.

2.4GHz has max 20% speed of wired.

5GHz has max 50% speed of wired.

I stand close to the router to test the speed.

Even set the channel to auto, it wont solve the problem.

1.1.2 Build 20210125 rel.39623(4555)

Having same issues, worked great on last firmware but since last update wifi speeds have dropped from 300 down to about 180-200

Tried to downgrade firmware to last known good version but it says file invalid.

Jerry