AirPlay doesn't work when AirPort Exp. extends my network? I have recently acquired an old first gen AirPort Express and wanted to use it solely for airplaying over to my old and amp.

I tested configuring with it's own network and works fine. The problem is, when I ask it to join my house's network, which is not an "Apple network" and is composed of a few routers extending it. The AirPort still shows on my preferences, is detected by the AirPort Utility but doesn't seem to be reached displaying "Error -6753".

I am figuring there is some port configuration I am ought do (?). I don't remember configuring internal firewall in any of my routers (except for the one hooked to the modem), but maybe this is a known issue?

[EDIT] Some more details on my equipment:

I have the ISP's modem, connected to a cheap Huawei wireless router, which has a firewall configured to keep outbound traffic away.
My house has an elongated shape, so midway through the rooms there is another Asus (RT-N10+) wireless router, configured to extend the Huawei's network (configured simply as bridge, no firewall, mac filter whatsoever).
The AirPort Express would be in the lower floor, still, closer to the Huawei router (thus what I believe it would connect to).

The network itself is SSID broadcasted, WAP/WPA2 Personal Encripted.

I will be trying to AirPlay from iPads, iPhones and a MacBook Pro over the house, which will be connected either to the Huawei or Asus router.
It will work wirelessly if you are able to get Wifi from some of the old routers. In other words: you need Wifi signal from your existing network for both your AirPort Express and the Apple device to use AirPlay in this configuration.

The Airport Express will work on client mode and will not extend your network, but it will allow AirPlay. This is because AirPorts do not wirelessly extend non Apple networks. See this question for reference.

I have a similar setup in my home, and my solution was to run an Ethernet cable to the AExp to be able to use it to extend my network, and do AirPlay.

On the Error -6753, I've found someone suggesting you have to enable IPv6 on your router.