SKX80 5G wifi drop to 802.11a

Hi,

I mainly use wifi 2 and wifi 3 with the SXK80 and 1 satellite setup. So I disabled the wifi 1.

When i first set it up, the 5G wifi connection is 802.11a (54Mbps shown on samsung phone and 802.11a shown in win11 laptop).

I thought it was because I used a bad cable for WAN port which it only give 100M connection speed. After I changed to another cable, it is 1000M now, and then the Wifi speed is back to normal at 1.1Gbps. 

Then I added the router to the insight app last night and it said the settings would be cleaned etc. But i found that the settings e.g. port forwarding remains.

This morning I have found that the wifi speed went to 802.11a again. (without wifi6 shown on phone and only 54Mbps)

I then tried to enable Wifi 1 again, and the wifi speed back to normal.

May I know the rationale behind all these? 

Thank you.