IPsec VPN Fortclient - auto-connect doesnt work when is on APN network,

Hi guys,

My ipsec vpn is working normally including features like: auto connect, save password and always up. but if I establish the connection between fortigate and forticlient via APN the auto connect functionality will stop working. that is, the auto connect functionality only works when the connection is established over the internet. how can i solve this guys?