Matchmaker issues across networks

I've been experiencing problems locating matches using the Matchmaker across different networks. It appears that matches created by a client are only visible to other clients on the same network. I'm using Unity 5.1.3p3 (I've also tried 5.2.1f1), and the stock NetworkManager and NetworkManagerHUD.
I've been testing by creating a match on PC then trying to connect using the same client installed to a Android phone on a different (WiFi) network. If I run two clients on PCs on the same network...

Matchmaker issues across networks