Cascaded network Airplay issues

Hi. I have a cascaded network, which means I'm running an Airport Extreme as router #1, and a D-Link DIR-615 router as router #2. WAN on router #2 is connected to LAN on router #1. In this way, I have two different subnets, which share the same internet connection. Subnet #1 uses 10.0.1.x IP, subnet #2 uses 192.168.0.x. Mostly this works fine. However, I have a few Apple TV's around the house, which are meant to work on each individual subnet. Here's the problem: Sometimes, but not always, the Apple TV's on the other subnet shows up as available Airplay devices on my mac or iOS devices. I can't seem to actually connect to these, and they don't appear all the time.

How is this possible across two different subnets, and how do I prevent this behaviour?

For reference: I haven't figured out why this happens, but I have found a workaround: Apple used port 5353 for mDNS, which is used for Bonjour to work across different subnets. I set the second router to block that port, and haven't seen the issue since.