The other risk to that is they’d possibly gain access to your internal network through your NAS. No telling what a bad actor would do.

A white one.

You could probably use any off the shelf consumer wireless router that supports OpenWRT and dual band.

Setup the 5Ghz side to connect to whatever the network source is, and broadcast your own SSID on the 2.4GHz side. I’m currently doing this same thing with a TP-Link AC1750.