Hi Martijn, I think I found the reason why it’s not working.
Eufy support says: The Floodlight Cam E340 does support RTSP/NAS when it operates independently. However, if it is connected to HomeBase 3, the RTSP/NAS function will not work at the moment. The RTSP feature on the camera is compatible with NAS devices that support the RTSP protocol, and Synology NAS is confirmed to be compatible with eufy Security. For other NAS brands, compatibility has not been tested by eufy.
In my case I have the camera connected to a Homebase 3 so now I know why it’s not working. Thought I might share this for others that have the same set-up. Took me some time to figure this out.