2 modems likely means 2 separate accounts which is roughly $50-$100/month extra.
Your realistic options are:
1 add a wireless receiver to teh upstairs computer, and hope the router covers the range.
[the usually better way to do this is buy a cheap wireless router that supports natively or through third party firmware the ability to act as a wireless to ethernet bridge. As routers are generally better at both receiving and transmitting than the usb addon cards they sell. You can also by an internal card to add to the PC, but those are going the way of the dinosaur and probably hard to find these days, plus probably older tech.
2. Try moca over the coax.
3. Down and dirty hole drilling in walls to run the Ethernet yourself. Just be sure to get some sort of sealant to fill the holes after you insert the cable through them. Some some of plastic clipped attachment for the outside of the house to run the wire around it, and a detectoir that can find power wires before you drill.