[morphios]

I see.

I'm still a bit curious as to how you "let them load their settings".
Is it somehow via remote desktop, do you literary unplug the router and hand it to them for configuration, or are they using another method altogether?

I plug the ISP cable into the WAN port and call them and they access it remotely on their end.

Obviously that's how ISP's provide remote access support.

I have to disagree with you here.
Maybe my brain is just wired different, but I cannot see that answer as "obvious". The answer depends a lot on where he lives, what type of infrastructure is available to him, and how it is all set up by the ISP.

If DLS was still used, he cold not merely plug in his communications line to enable the ISP to access the router, apart from the fact that the ports are completely different. Not that I haven't seen people try, though.
Just like DSL, every type of connection has its own type of setup and by telling me he just plugs in a cable to enable them to access it tells me there is already a connection on that cable that he is plugging in.
The type of connection is currently undetermined.

For instance, if the cable he plugs in to the router is plugged into a device that has a network port and that device has a static IP in the same range as what the default IP range is of the router when reset, and it provides you with internet access on that device, there will be no point trying to configure anything on the router itself because the router then only act as an access point and then the initial port forwarding happens elsewhere and will disregard anything that is trying to establish a connection on a port that is closed.

Only when it is determined how the infrastructure works, one can decide how to manipulate it.