Archer C5400 Wrong Update of Wrong Build causes Speed issue!
I was using 1.2.6 Build 20220511 rel.37896 firmware update. My speed always seems to be 500 MBit at the router speed test, but after 10-12 hours it drops to 250 at cable and Wi-Fi but still 500 MBit at the router test. QoS and other settings didn't help this issue. I looked at firmware updates and seemed 1.2.5 was fixing the "The homecare module is optimized to solve the problem of speed issue in a specific environment" issue.
As a developer and computer engineer, I am guessing this is an issue for the firmware developing side. They probably jumped 1.2.6 from 1.2.4 or another old build and missed this issue fix because this issue returned at the new update.
At least it's allowed to downgrade firmware I am returned to 1.2.5 and my issue is solved, even though my speed seems 650 now! Never going 1.2.6 again!