Well, you could:
4.) run ethernet cables connecting the router downstairs to the computers up stairs (or run only one and connect it to a switch upstairs that connects to both computers). But maybe you don't want to run the cable.
5.) relocate the modem, router, and wireless access point upstairs. It might save you buying a couple of wireless network adapters, assuming all of the computers downstairs are using wireless connections already.
I don't recommend powerline networking if any of the other options will work, as they will be significantly slower than the other options, even if you have good wiring. See
this review for comparison.
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