Thanks, this was the option with the least effort, so I tried it and it worked.Ok my take on the situation, I’ve had a similar experience when I wired up my sons room with a pair of cat5e cables around 20m in length, any device connected to either would blip 1Gb then drop down to 100Mb and what I found was that my HP 8 port switch just couldn’t properly drive the cable and maintain a reliable 1Gb connection so it would drop.
In the end I replaced the switch with a Dell 16 port device which I just happened to have and both pairs now reliably maintain 1Gb.
It’s not clear how you have the devices connected ie direct via a x-over cable or via a switch but hopefully the above might give you something to test?
What I did is to connect other Gigabit router to the same network port with which I had a problem.
I've connected the "problematic" cable to the new Gigabit router and the issue was resolved.
The router with which I had problems is Hauwei AX3 Pro 7200.
It is a WIFI6 router with 4 Gigabit ports.
Guessing the router OS, is a bit buggy, since the network port itself works.
Thank you all for all asistance.