Netduma R1 problem with Fiber and COD

[CODduma]

Hi, I play COD on PS5. I recently bought a Netduma R1 because I was experiencing severe desynchronization issues in FPS games on my 1Gbps fiber connection.

The R1 is running the latest available firmware, and I’ve configured it correctly following the forum recommendations.

Here’s the problem: When I connect the R1 to my ISP's router and then the PS5 to the Duma, I can launch the game but can never connect to a lobby—I get a "Host Unreachable" error. However, as soon as I connect the R1 to a 4G hotspot via Ethernet, I find lobbies almost immediately. Have you ever encountered this situation? Why doesn't it work when going through my ISP's router?

I’ve tested both with and without placing the R1 in the DMZ on my ISP's router.