bonjour/mdns repeater?
If you have a Linux/BSD computer connected to both the networks, you can use the mDNS responder Avahi to reflect the bonjour traffic between the two networks.

The relevant Avahi configuration (avahi-daemon.conf) is a section like this:

[reflector]
enable-reflector=yes


Here's another relevant SE thread with answers:

What, exactly, is required to make Airplay work across VLANs?

I have successfully used Avahi on my Soekris router (running FreeBSD) for several years to solve this problem.