FGT 60D - Interface Down but still responding to ping

Hello all!

I need some advice. I have an odd behavior on a FGT 60d 5.4.2. WAN1 interface is down but it still respond to ping requests. I tried to access it with HTTPS and no luck. I can access the FGT from the LAN 1 but when i connect our MPLS ISP router its IP starts to respond along with the WAN1 but WAN1 stops responding after 5-6 pings and interface goes down, router responds fine thou. And since my routing rules on my main office is dependent to the router IP, once it is connected and responding my internal network stops responding because my main office routing rule is expecting communications coming from wan1 and not throu our backup VPN.

Can someone give me a light here?

Regards,

hervaltelecom