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I have a pfsense CE firewall, free, on an old mini desktop. The web-configurator has a traffic graph status page that lists every connected device's bandwidth in and out, updated every second. An overall LAN/WAN traffic graph too, of course.

There's these little wifi extenders that you can plug into an outlet on the main floor. They're a dime a dozen in the big cloud stores. Find a first floor location spatially about half way between the 2nd floor wifi router and the basement spot you'd prefer to work, and plug it in and get it connected to the main wifi.

Whatever you find, try and match it to the main router's wifi standards and bands. Or get even higher-rated repeater for longer term compatibility with upgraded main routers.

Assign a static IP address the nic at each PoS computer with a bogus gateway ip address and no DNS server entry.

That computer won't find the internet but still will communicate on the local lan.

Then add hostname/address pairs in its HOSTS file as needed.

Starlink. Or its LEO ISP competitors like Amazon, within a few years.