TLDR: Any standard PC with two or more Ethernet interfaces and OpenBSD with PF.
I like the small Qotom Q305p 3205u computers for this, but any small device will do and if you don't care about the router being small and quiet, any form factor PC will work... just need two or more NICs to route packets from one network to the other.
I like the small Qotom Q305p 3205u computers for this, but any small device will do and if you don't care about the router being small and quiet, any form factor PC will work... just need two or more NICs to route packets from one network to the other.