First, I don't have an AC-66, but I believe this is an issue that goes further back and also effects RT-N66s. I have no idea how Asus defines their media bridge mode, or exactly how it works. I have 2 x Linksys APs running DD-WRT in bridge mode on my network, and my AppleTV is seen behind them OK for AirPlay. There was another guy on here just the other day that I was posting with, who I believe used DD-WRT in its bridge mode instead of Asus's native bridge mode, due to problems similar to yours.
I also have 3 TiVos, 2 of which are behind my Linksys bridges. Both of my bridges are running on 5ghz band (though used to have one on 2.4ghz), yet I just noticed I have IGMP snooping off for 2.4ghz, but enabled for 5ghz. I mention this only because TiVos also use a lot of Bonjour/aka Apple Zero Config to talk to each other. All 3 have been able to see themselves no matter how my IGMP snooping was set, and can still see each other now.
The following two points I'm just throwing out there for people brainstorming how/why some media bridges work and some don't, not necessarily asking for help on my DD-WRT issues or expecting answers in this thread (though feel free to PM me if you'd like!
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- For the record, I'm going to throw in that though I have IPV6 address to my router and many of my machines, that DD-WRT doesn't have IPV6 fully up and integrated yet, so my two bridges aren't passing along IPV6 chatter.
- I can't see MAC addresses through the bridge (at least, on the bridge's side of things). This is a known DD-WRT thing and I really can't quite figure out what's going on, the "friendly names" I've assigned the MACs behind the bridges in my DHCP reservation list actually show up in the list of connected devices, and sometimes it will even correctly ID the bridges as "routers" w/ the IP addresses I have reserved for them (I'm talking about just in the list of connected devices). So, even though it looks kind of opaque, it actually still works just fine and I don't have the WDS problems of mandatory WEP encryption and more or less 802.11G speeds.
I'm playing around with Wireshark on a desktop that plugs into a switch, that then plugs into my RT-N66 (so I'm not even sure if the traffic that I'm seeing is of any value), but I did find it interesting that I got those lines in red regarding MDNS (Bonjour) the first line is something about my NAS talking to the AppleTV, and the second red line is my AppleTV's IP.
Then a few lines down (highlighted w/ cursor, line is bright blue) 2 of my Tivos are asking to find each other via ARP and it was one of the bridges that was doing the asking.
And I do have spanning tree on, I'm not sure if I need it, but I figured maybe I should since I have a router/AP, 2 x bridges, and an auxiliary AP on my network.
To OP, sorry this probably doesn't answer your question (except maybe to give it a try by loading DD-WRT or other f/w on your bridged router?), I'm more kind of documenting some weirdness of bridges in general.
If anyone thinks this post goes too many directions let me know and I'll move or edit it.
Thx