You can forward ports through multiple routers/modems but it requires extra steps. There's a couple ways to do this. You could forward the ports from the modem to the router, then from the router to your computer, or you could put your modem in DMZ mode then forward the ports from the router to the computer.
There should be an IP somewhere on your modem with a login and password, you can type that into a browser to log in and forward the ports from there. Depending on what brand and model it is, the interface is going to look different but there are guides online that show you how to do it on just about any model. Keywords to look for are NAT/Gaming/Pinhole/Port Forward.
You should be able to do the same on the router to forward...