El Capitan "Create NAT64 Network" - connection issues

If I get your network configuration correctly, your setup is completely hosed.

You need two separate (physical) networks:

• one with IPv4 only (e.g. ethernet)
• one with IPv6 only (e.g. local Wi-Fi)

Depending of the capabilities of your machines, the machine with two interfaces (ethernet and Wi-Fi) should be the server (DNS/DHCP etc.)/NAT64-router (= macsrv).

Since the iDevices don't have an ethernet interface and you try to test their IPv6/IPv4 functionality they should reside in the Wi-Fi section, which should be dedicated to IPv6 then. This local Wi-Fi network is not your Fritz!Box' Wi-Fi! You have to setup a personal hotspot on the NAT64 router!

To access a IPv4 server from your iDevices you may either use your normal router connection to the WAN or a dedicated IPv4 "server" (= macmedia) - both of them reside in the ethernet section "behind" the NAT64-router (= macsrv). If macmedia doesn't have an ethernet interface just connect it to the Fritz!Box Wi-Fi, because technically the Fritz!Box Wi-Fi and the Fritz!Box ethernet are both in the ethernet section.

To access an IPv6 server use a web server on macsrv.

Since you don't need IPv6 on your Fritz!Box router, simply turn it off.