When you say 'modem' do you mean a combined modem-router? Brand?
If you have a modem-router, you can set it to 'bridge mode' (google it) and connect a second router to it, but then you would need to connect your pc to the second router.
If you leave the modern-router operating as a router (so you can connect your pc to it) and just connect a second router to one of the ethernet ports (and also use this second router as a router) that set up will kind-of mostly work some of the time, but you'll run into problems (occasional IP conflicts and so on; you've got two dhcp servers running amongst other problems).
Alternatively, if you can just set the second router to 'access point' mode (so it runs without it's own dhcp etc), then yes you can just plug it into the main router/modem in the bedroom (and plug your pc into that).