Dear Susan et al:
I've seen lots of versions of your question. One answer seems to be that you can use the Comcast device to connect to the Internet and manage your indoor wireless network. You can still apparently use your Airport Express as a wireless "bridge", essentially just re-transmitting the Comcast network signal in a different part of the house. That would seem to address your identified weak signal problem.
I think this can be done by plugging your Express into an electric socket in another room, then using your computer's "Airport Utility" program to reconfgure your Express from being a router to being a "bridge" retransmitter.
I may be wrong, and you might need to run an ethernet cable from the Comcast device to your Express in another room. But you could try the above first.
Caveat:
My login name above is "davidrAppleSupport", but I don't work for Apple Support.