You may have seen the following message after launching Ecamm on macOS Sequoia:
This permission is required for Ecamm to work with RTSP cameras as configured in the Network Cameras window
If you’ve seen this prompt and have chosen “Don’t Allow”, you can grant this permission by enabling the toggle next to Ecamm in the Privacy & Security control panel of your Mac’s System Settings application. You can find this under “Local Network” once you have selected Privacy & Security from the left.
If you're still having trouble with RTSP cameras after double-checking that this permission is enabled, check to make sure you are running Ecamm from your Mac's Applications folder and try again.