Using TimeCapsule through PFSense firewall
Okay, that was quite tricky. Like I've said in a comment, Bonjour is used by my Time Capsule to be recognized by the mac but isn't supposed to work on a different subnet. 
I set up an Avahi server on my pfsense router that is forwarding any Bonjour request to the WLAN and I can now access my Timecapsule by wifi, use AirPrint and Airplay. 
For more informations : http://www.avahi.org