1, Confirm you camera rtsp stream.

The url like: rtsp://username:password@ip:port, confirm it by VLC etc.
eg, TpLink camera: rtsp://admin:admin@192.168.00.00:554/stream1

2, Install Homebridge APP to Synology NAS.

3, Install "Homebridge Camera FFmpeg" plugin in Homebridge APP.

4, Set your camera in "Homebridge Camera FFmpeg" plugin.

Video Source: -i rtsp://admin:admin@192.168.00.00:554/stream1
(-i rtsp://admin:admin@192.168.00.00:554/stream2)

Other

Unrecognized option 'srtp_out_suite' error

The reason is the plugin is using the old version ffmpeg. You can change the ffmpeg setting in Global Advanced > Video Processor.

How to install another ffmpeg

It is not recomended to upgrade the NAS default ffmpeg at command line, you can add some package source which is contains ffmpeg package, and then install it in your package center.
The new ffmpeg binary file path is /volume1/@appstore/ffmpeg/bin/ffmpeg. You can just add the path to plugin "Video Processor" setting.
Of course you can download ffmpeg at NAS command line and then set the "Video Processor" point to it.