In my case the cable was faulty. I was not knowing about it until I rechecked today. With a correct cable between AX55 and desktop, the network speeds got restored. I am not facing internet speed...

For most questions, the answers are in the screenshots as evidence. PPPoE is used on the ODN box.

The drop in the internet speed is quite evident from SpeedTest on Google or Ookla. Information and test results below with and without AX55. TP-Link AX-55: DNS on latest CachyOS desktop: Only wired...