That's right. windows network discovery uses multicast or broadcast packets (I think multicast). There was a lot of hassle we did to set up something like that across subnets on one thread a few years ago. it turned out even if we got the packets routed correctly windows refused to accept them because they weren't from the same subnet. (I found the thread )
Easiest thing was to simply create a printer and mapped drive using the ip addresses or names directly.