Firstly, as I mentioned on this topic (https://community.tp-link.com/en/home/forum/topic/583774?replyId=1195010) in the community, version 1.2.6 brings back the speed drop issue that was fixed in 1.2.

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 a