It depends what kind of isolator they have installed in the wallplate. If they installed one that's TV and FM, there will be a bandstop filter on the FM port which makes it unsuitable for use with a cable modem. In some cases, they install one that's just a simple splitter with no bandstop filtering that would work fine. Sadly, it seems your'e out of luck. As a test for this, temporarily disconnect your STB's cable, and connect your cablemodem to the TV port. It should work fine.
It's possible you could get a splitter like the one they install in the brown box outside (they only pull one cable to your house - usually) and wire this to the TV port, and then wire your TV and CM to it, but that's getting past simple, and the insertion loss of the splitter would probably stop it working.
Time to think about getting wireless perhaps ?
