I've asked Comcast/XFinity about this a couple years ago and they specifically said no. They do not support multiple modems in a single residence. Something along the lines of having to run a second line into the house was the excuse I got.
I recently moved into a similar situation. The modem/router on the main floor, South end of the house and I'm in the basement on the North end of the house. I tried WiFi and kept disconnecting. The next option would have been a WiFi extended, but I managed to figure out how to run an Ethernet line to my location.
If you have a coaxial output in that room, you may be able to trace how that line is run and run Ethernet along it like I did. Mine was mostly exposed, but it's well worth a look into the attic, garage, storage spaces, etc... to see if it's viable.
As a last resort, you can look into options like PowerLine and Media over Coaxial Alliance (MoCA). These will generally (but not always) work, but also have a much higher upfront cost.
150 Ft Cat6 Ethernet
Ethernet Data Plug (50 pk)
Ethernet Crimping Tool
Ethernet Wall Plate
Zip Ties (20 pk)
Total ~$75, a lot of sweat and some elbow grease, but I'm fully wired
-Wolf sends