BLF Subscription issues

We are having some issues integrating our Asterisk server with Grandstream 2140 phones and the BLF indicators. Everything else works great but the BLF indicators timeout and never resubscribe. Has anyone experienced this before?

We are running Asterisk 11.18.0 with Grandstream firmware We also have Netgear GS728TP switches(turned off auto-vlan functions) and a Fortigate 40c running version 5.0 build 0292 firewall.

The phones connect and blf works fine on reboot but after expiration they never renew. When running a SIP SHOW SUBSCRIPTIONS right after reboot we see the ip as the phone ip but after subscription it changes to the IP of the Fortigate gateway.

Any ideas??
Thank you in advance!