VPN works with LTE but not with my ISP

Hello,

I have a weird issue with a VPN service.

El Capitan 10.11.1

OS X Server 5.0.15

It works pretty well (L2TP) when I use my iPhone's LTE connection (via Personnal Hotspot) with any computers. But the connection doesn't work when I used a normal wifi connection via my ISP.

Nov 24 10:58:59 *** pppd[56115]: L2TP connecting to server '***.net' (xx.xx.xxx.xxx)...

Nov 24 10:58:59 *** pppd[56115]: IPSec connection started

Nov 24 10:59:09 *** pppd[56115]: IPSec connection failed

LTE : Telus

ISP: Videotron

All VPN required ports have been forwarded on the router. (D-Link).

All other services are working well (Calendar, File Sharing, DNS, Contact, Web, Wiki...).

I deleted the /Library/Preferences/SystemConfiguration/com.apple.RemoteAccessServers.plist with no success.

First time I see that issue as I did configure a few OS X Server in the last year.

Any clues?

Thanks!