Hi everyone !
First let me show you an image to sum up the situation :

I would like to get the purple link to work !
So basically I have an openvpn server running somewhere in france with IP AA.BB.CC.DD and I am located in an engineering school which has pretty restrictive network(802.1X...+proxy). I have a TP-link WR1043ND running openwrt in my room on which I run Wpa supplicant to connect to 802.1X and an OpenVpn CLIENT to get through the school proxy.
I also have a static route that redirects all 129.104.0.0/16 to the school network via 129.104.224.254 (gateway) so I am able to reach some local awesome services without disconnecting the vpn client.
I need to reach these services while I am out of the campus i.e to fix the purple link on the image.
I am already able to ping the router IP with a computer connected to the vpn anywhere.
The server runs on 554TCP and has client-to-client enabled. all other settings are default.
I tried to create some routes and to use iroute but everytime I broke the entire setup so I finally crash here... :-)
Thanks for your help !