(I'm not an expert with this sort of stuff, so forgive me for any wrong terminology etc.)
For reference, here's what it is, 6th and final generation specifically.
Basically we mainly use it to expand the wifi coverage on the property, since it acts as a wifi hotspot and allows us to create a 2nd network/connection for us all to use and in addition to our original modem/router.
It's been on the fritz lately, was discontinued several years ago, and it doesn't seem like there's anything exactly like it anymore. Any suggestions? Ideally something cheap and easy to set up.
My setup is an old PC running OPNsense wired into a bunch of OpenWRT access points.
That thing is just an access point or a "dumb" router. You can repurpose just about any low range wireless router to give up doing routing and just relay signals. Either wired up directly to the main router or you can have it create a wireless connection to an existing AP that's closer to your router.
Here's a list of compatible routers. Old Asus ACXXXX devices seem to work pretty well.