To see if your router/modem is running a vulnerable version of RomPager, run Wireshark and access the device's configuration page.
Look for a HTTP/1.1 200 OK packet, inside that will be listed the server version, eg:
Server: RomPager/4.07 UPnP/1.0
Which suggests my modem is vulnerable, bugger. Time to get a different one methinks. Good job it's separate from the router.
Then again, would my modem even be reachable from the internet if it's running in PPPoE bridge mode? Wouldn't the PPPoE-encapsulated packets get sent straight to the router without the modem even bothering to look at them?