@Tony its a cat8 ethernet cable and the device is a laptop so neither of those are the issue...its a problem with the deco and their firmware..as per other people with the same issue here and reddit.

I have a 3 pack of M5 Deco setup but when I connect via ethernet to the second port on the main deco the speed is capped at 100MB. The internet connection is capable of 200MB and this can be got over...