Issue : NAT Loopback (hairpinning) | WAN Access routed mode

Hello everybody, 

I am currently experiencing a problem that is quite blocking for me. I will present the context and the manifestation of the incident.

Context WAN : 

--2 WANs access : 

- WAN1 : ADSL PPOE --> IP Public know by fortinet 

- WAN2 : 1 VDSL routed mode with the provider modem (No other choice with this provider) --> IP Public unknown by forti (Just Private subnet of routing, and 2 interfaces)

After i have been applied this process : https://cookbook.fortinet.com/using-hairpinning/. The hairpinning works great for the 

ADSL (IP Public on the interface WAN1).

Issue : On the WAN2, the NAT loopback can't work because the forti can't map the interface with the real @IP_Public (my opinion)

Question : 

Is there a way to tag the real @IP_Public on the WAN2 without changing interconnect mode (routed with modem) ?

Or an another process to bypass this issue and permit loopback on the 2 WANs ? 

We have considered the solution using DNS cheating on the internal DNS, but may be a source of others problem.. 

Thanks you in advance for yor help, 

Have a nice Day

Brice