I see nothing wrong with your settings. The 'Enable NAT mapping protocol' is what it needs to work for UPnP. Have you tried resetting the AirPort? I've never reliably had it working on any of my Macs in the last few years, but I don't use Airport routers anymore.
Hmm ok. Must be the beta then. Just wanted to confirm.
Not a feature I NEEDED right now, however I noted that it was in fact working on my 2015 5k iMac with the latest EL Capitan on it. And it was good for those accidental "I left the file at home" days.