On Friday, August 21, 2015 at 9:52:51 AM UTC-7, ceg wrote:
I just got a new home broadband router that has both 2.4GHz & 5GHz
and only then did I realize that both the Windows and Linux laptops
don't have a 5GHz Wi-Fi card (both only have 3.4GHz 802.11 b/g/n).

Neither has either 5GHz or 802.11 ac.
What are my options?

Use your router with your Wi-Fi cards.
If it bothers you that someone else might use the 5GHz connection,
go into the router administrative settings and disable it. There's nothing
wrong with 802.11b/g/n.

The laptops needn't use the wireless at all, you can (and probably
in an apartment with lots of neighbors, will prefer to) use a
wired connection from the main machine's desk to the router.