If your lines are home run, replace the splitter with a barrel connector
so that the incoming comcast line is patched straight through to the line that goes to where your modem is.
Then your other now disconnected cables can all be tied together with a splitter that is completely separate.
If you would try to mix the two, the antenna would pick up so much noise that your internet would not work. Also the noise would backfill into the Comcast system and could disrupt your neighbors. Additionally Comcast signal could find its way out your antenna and create a huge cable leak. It would be found by Comcast techs running egress and they can disconnect your service at the tap when they find the point if ingress and egress is coming from your house.