Well if you don't have access the router, ipconfig /all from machines from each company should tell us if your different segments or just same one, etc.
You don't have access to the router why? You don't know login, don't know the IP? You should be able to console into it and get the config. Did you try just default username and password?
don't know the login AND the IP to it. why? because i don't know (and i'm pretty sure no one does in this client ) who's the owner of that router: the ISP's, the client...or else I'll just plugin a console cable and recover the router buy resenting the credentials, but i can't since i don't know who's the owner of that.