@Kevin_Z Thanks. I'm sorry but I don't think I made myself clear enough, the issue wasn't there before I switched to the Deco network, I could always access the cameras in Apple's Home app (as they're on Homekit Secure Video), and there's no option to set remote access, as that has always been the case (also before Homekit Secure Video, when I could access them remotely in the Logi Circle app). So the issue is clearly caused by the Deco network.

@Kevin_Z I also cannot access my Synology NAS (which is on a different network) from the Deco network, but I can access it remotely from my phone when on cellular, making me draw the conclusion that the issue lies with the Deco and not the NAS. So any idea how I can solve this?