Hi FA, I've some general ideas. Others will have better comments on your particular questions.
Also the ethernet-dhcp example is copied to /etc/netctl/dhcp and edited for the correct interface (enp0s25).
Unless the remote hands are used to precisely editing configuration files, this might be something where a mistake has been made.
I could probably find out what is wrong if I had my hands on the keyboard, but doing it remotely seems more difficult...
Can they email you photos of their screen at interesting points? Or stream a video of it as you're talking to them? It might lessen the friction of being remote enough that you get through more and diagnose the problem. Would a KVM be useful? Could they get hold of one temporarily? https://pikvm.org -- Cheers, Ralph.