This is not how I have my FreePBX test box setup behind a Dynamic WAN IP using DDNS FQDN and I have no warnings on my WEBGUI Dashboard… Im not even sure why you would add your FQDN (Public WAN IP) to the Networks Trusted Zone either…
I run pretty much the same setup as you for 1 of my FreePBX test boxes with a VerizonFIOS DynamicIP and also use DDNS that autoupdates via the update module built into Untangle Commercial NG Firewall. Same setup as you just using Untangle instead of SonicWall… VerizonONT > Untangle NG Firewall> 48 Port Switch> Configured VLANS>VOIP VLAN>FreePBX sits on VOIP VLAN.
Your PC that your logging into the FreePBX webgui from… where you’re seeing the non-trusted message about the PC your using to access the webgui… Is it on the same local LAN subnet as your FreePBX box? Or are you accessing the FreePBX webgui remotely?
If youre accessing the WEBGUI from your local LAN subnet, you need to add that subnet (or at the very least the IP address of this computer) to your Networks tab and set to trusted… Your PC behind SonicWall is assigned or set statically a LOCAL LAN IP… It doesnt get your PUBLIC WAN IP which is why your still seeing that message…
Now, if your connecting to the webgui remotely, then thats a whole different setup. I use Wireguard VPN, and the VPN IP Subnet that Wireguard hands out to connecting VPN clients is set to Trusted in Networks and Whitelisted in Intrusion Detection and I also dont get any untrusted messages in my WEBGUI dashboard.
All of this is setup as outlined in the FreePBX Security Best Practives WIKI