I don't understand what you mean by:
the faster of two connections is inaccessible
Do you have 2 connections from Comcast?
I would troubleshoot your connection before calling.
When I have an issue I:
1. Remove any splitters from the path, then check it again. If that doesn't work try step 2.
2. Connect the modem directly where cable comes into the house and then hook up a CAT5 cable directly to a computer and check it then.
If you still have slow speeds after step 2, it's not your problem. Call Comcast then.