Access remote clients through router

[sjdrc]

I'm trying to figure out what what sort of settings I need to change for my use case.

I have a remote server (A) acting as a wireguard server, my router (B) is a client, and a different remote server (C) is also a client.

I would like to be able to access server C from a computer that is not on the vpn behind router B. What firewall settings and static routes need to be set to achieve this?