If I have multiple Coax Cables, can I have multiple modems and routers? (video walkthrough attached)

I have an "active" coax cable in my living room. That is, when I plug it into my modem, I get an internet connection. I have several other coax cables wired throughout the house, but none of them output an internet connection.

I'm trying to connect a modem and router in every room so I can get a strong connection throughout the house. Right now I'm running a network mesh, and the connection is much weaker by the time it hits our office/rooms.

Here's a 4 minute video walking through the exact network setup:

https://www.loom.com/share/663281a982cb4536bb0e5a8748774236

As shown in the video, I have access to the garage where all the coax cables connect. I'm not sure how to debug whether there's something I can do there. I would love to figure this out, but 2 questions:

1. Is what I'm trying to do even possible? Can I have multiple modems/routers hooked up in each room?

2. If so, am I better off having an Xfinity technician come through, or is there something easy I can do on my own?