So i your gateway modem in that panel place in your garage? then you ahve cat 5 running to your house that you plug into for internet?
how many coaxials lines are running into your house from the garage panel?
Depending on signal degeneration you can move you gateway inside to one of the 6 comcast coaxial and it should work. If you have Ethernet in the house from the comcast gateway. Why don't you get a new wireless router and plug it in the ethernet line inside?
do your phones plug into a device or a wall outlet? if device what is the model?
Could you give me the model numbers on the cable modem. is it possible for you to post a picture of the box in the garage and the coaxial to eithernet device. Comcast telephone is VOIP to analog. So the voice once it leaves the box on the copper wire is analog. It's like the old phone where you can have multiple phones hooked up to one line. If that makes more sense.