I'm not suggesting a solution as much as I'm hoping to clarify some things for you and consolidate some things into one post so that those offering solutions might take note of your comments about homeplugs. (And homeplugs can have crappy throughput.)
For the purposes of this post, a modem is one of the first pieces of networking equipment that data coming from the internet passes thru in your home. A router is probably the next piece of equipment in the path. The circuitry that functions as the modem and the circuitry that functions as the router can be in the same box... it can appear as one device on your home's network.
....I know my ISP doesn't provide static IPs, will that be a problem?
Network traffic involving the internet is on what I'll later refer to as network #1. There are no static IP assignments needed for network#1.
I'm going to guess that your modem is getting one "internet facing" (dynamically assigned) IP address from your ISP and that the router has its Network Address Translation (NAT) function turned on.
That NAT function takes that one ISP assigned IP address and splits it in a way that lets all of the devices on your home network connect to the internet (if need be). Or, to illustrate with made up numbers: let's say that your ISP assigned 183.218.23.85 to the modem - then the router assigns an IP address of 192.168.0.100 to one of your computers and 192.168.0.101 to another device on your network. The router keeps track of which device inside your home has asked to send/receive data to/from the internet. It routes the data to the correct device.
....The router is upstairs, the NAS is downstairs,....
One of the suggestions was to plug the NAS directly into your router, but then you mentioned that the homeplug Ethernet adapters have a bit of a struggle with that much data going thru them... so the data slows down when it hits the homeplug that is connected to a computer. :-(
Hence your desire to connect the NAS directly to one computer without having a homeplug anywhere in the path.
So... if you continue with that plan - then
Computer #1 will have two network adapters installed:
One adapter that talks to network #1 in your home...
...& a 2nd adapter that talks to network #2 in your home.
Network #1 includes:
...one or more computers/homeplugs
...your router/modem/internet traffic
Network #2 will consist of only:
...one computer#1/PCI network adapter
...one NAS
When computer #1 wants to talk to the internet - that should work just fine. If it does not, then there are some things to try. When computer #1 wants to talk to the NAS - that too should work once you manually assign an IP to the PCI network adpater inside of computer #1 and manually assign an IP to the NAS
I'll assume that there is a computer #2 as part of your home network. I'll also assume that you want computer #2 to be able to access data from the NAS. Correct? That might take some more adjustments to computer #1 to get it to serve up files from the NAS to the other computer(s) or devcies on your network.
In your current network setup, data traffic going from the NAS to computer #2 will travel thru two homeplugs:
NAS > homeplug > homeplug > computer #2
In your proposed network setup, data traffic going from the NAS to computer #2 will still travel thru two homeplugs:
NAS > PCI NIC > homeplug > homeplug > computer #2
One thing gained by your proposed network setup is better data throughput between NAS and computer #1.
I hope that this post helps others to help you...