Thanks for your help I made a ticket.
After watching closely all the thinks I need for my ISP there is something else that may be the cause.
I need to Tag CoS (802.1p) - with priority 6 so the dhcp request come first.
Is it the same thing as putting QoS to 6 in my wan settings ? For me it looks like the whole traffic is going with through priority and not only dhcp request with QoS at 6 but my network knowledge regarding CoS/QoS is very limited.