If you find a diplexer that can pass as low as 5mhz and blocks the DC on that leg it will work for you.
Your return frequency fall somewhere between 5-50mhz.
I'm using Perfect Vision diplexers and they work great for Comcast Cable Internet and Dish Network. Like the previous post said you need a diplexer rated 5-2150 Mhz and block the DC on the modem leg.
For troubleshooting purposes in the future, the best solution is to run separate runs. If you can have a new cable run to the modem that will be best if not just use the diplexers mentioned above.