@TP-Link deco p7 is in wireless route mode. The cable wasn't changed. However, I don't remember what kind of cable is it.

@Alexandre. Thank you for the hint. I already know that deco P7 initially negotiates 1000M, then in few/several days renegotiate to 100M. The entire point is why and how to prevent that. Both devices...

@TP-Link in short that would be my network topology: Speed test was done in deco mobile app. I assume it uses main deco device (P7 main) The frequency of the issue is every few / several days.

@1000bb I have exactly the same issue. My deco p7 after every several days switches from 1000M to 100M WAN port speed. The only thing which helps is turning off and then turning the deco on again....