Can't view IP DVR from WAN - Fortigate 60C

Hi!

Today I'm facing a problem with a FortiGate 60C in transparent mode placed between a Cisco RV320 router and a Cisco SG200-26 L2 switch. I have an IP DVR connected to the switch and want to view live cameras from outside. Port forwarding is set at the router as well as DDNS.

When I set up the device in our cam viewer software using DDNS, it appears as connected and available but when I try to view live cameras, video isn't showing at all and get an "device is offline" error although the device appears as connected at the device list. If I try to view recorded video or remote config from the DVR I can do it. Seems that problem is only when trying to view live.

I've added the used port (8003) as a service under objects at the Policy & Objects config page an also I've set the IPv4 policy from internal to WAN to allow but still no luck.

If I connect IP DVR outside FortiGate but still under RV320, remote access/view is working fine.

Find attached a couple of screenshots with RV320 and FortiGate configuration.

Hope you can help. Regards.