Route add to remote router

I have a dialup router at my site and I want a user to be able to use it from a remote location access via our WAN. I’ve tried various route add commands, but get the following error:

The route addition failed: Either the interface index is wrong or the gateway does not lie on the same network as the interface. Check the IP Address Table for the machine

Is what I’m doing possible? Logically, it sounds feasible and support article Q151795 implies it is possible. Any help is appreciated.

Greg