FortiClient IPSec VPN w/ VZW UML295

I still need to test this with the VZW modem UML295 in different environment before I can open a TT at TAC. But my initial test indicates somehow my laptop doesn't recognize the initial response from the server (FG1500D) when I tried IPSec VPN from FortiClient (5.2.????) when it's on VZW 4G network. The same IPSec VPN works if I put my laptop in LAN or WiFi environment. More strangely with the same FortiClient software on my laptop SSL VPN works just fine with the same server.

Did anybody experience the same situation?