None of the built in gateway modems have a dynamic DNS entry spot in them.
That doesnt necessarily mean you have to purchase a STATIC IP.
There are a few other options.
A ) You could put the modem in BRIDGED mode..which effectively turns it into a modem only (not modem/gateway), and continue to use a 3rd party router, which supports a dynamic DNS entry.
B ) many of the dynamic DNS services like dyndns and no-ip, offer SOFTWARE based solutions... that install on one of the PCs running at your location. Any time they notice an IP change, they will update your record on their server, pointing it to the new IP address. (i use no-ip and the software sits on my server which is always on).
C ) The rogers IP address.. usually only changes 1-2 times a year tops.. it should stay fairly consistant most of the time.. without a need necessarily for a dynamic DNS service... but yes there is a convienience to having it.
( D ) I leave this one seperate.. as i dont know WHAT you are connecting to, etc internally. If its just more for a remote desktop type session to a home PC.. you can use a service like logmein, for free, and no worries of having to remember anything. But that doesnt help if its for other things, servers, etc.)