@Dominik1973 

Hi, 
It seems your Synology and the Tapo C320WS cameras are not under the same network thus Synology can't add the camera successfully.

Please check if your network diagram is the Internet--ER605--Synology & AX6600 Wi-Fi router)))Tapo C320WS.

If so, since the AX6600 Wi-Fi router has the NAT feature to protect the internal network, you need to open the port (ONVIF Service Port: 2020, RTSP Service Port: 554) on the Wi-Fi router for the Tapo C320WS if you want to access the camera remotely.
If you want to put the Synology and the Tapo C320WS cameras under the same network, you can set the AX6600 Wi-Fi router to work in the AP mode.

How many Tapo C320WS do you have? Can you share the MAC addresses of the Tapo C320WS and the AX6600 Wi-Fi router with us via private message?
And please check if the 192.168.0.100 is your AX6600 Wi-Fi router's WAN IP address.

Best Regards

  @Dominik1973 

Hi,
1. You can refer to the link below to open the port on your AX6600 Wi-Fi router, here I use our Wi-Fi router as an example.
Port forwarding: how to set up virtual server on TP-Link wireless router?
Note: 
1) When adding virtual server entry/port forward entry, the internal port for the camera is 2020(for ONVIF stream), and please set different external ports for each camera to distinguish.
2) When accessing the camera outside the Wi-Fi router network, you can use the WAN IP of the router:external port number.

2. The below instructions may help you to set up the ONVIF stream.
How to view Tapo camera on PC through RTSP stream
 

Best Regards

  @Dominik1973 

Hi,
You can refer to one of the Cases in the link below to know the internal port and external port, there are the parameters that need to be entered when adding the virtual server/port forwarding entry.
Port forwarding: how to set up virtual server on TP-Link wireless router?
For your Tapo Camera, the internal port(port for ONVIF stream) is 2020, and you can set the external port between 1-65535, for example, use ports 900, 901, and 902 as the external port for your three cameras.

If you do not want to set the Wi-Fi router to use the AP mode, you can try to connect the Synology NAS to the Wi-Fi router rather than the ER605 router, then add the camera to the Synology NAS will be easier without open ports on the Wi-Fi router.

Best Regards

  @Dominik1973 

Hi,
Please connect a PC to the Wi-Fi router that the Tapo Camera connects to, and then refer to the link below to watch the camera via ONVIF stream using the iSpy program on your PC. This process is to verify the camera settings and the ONVIF stream of the camera are working.
How to view Tapo camera on PC through RTSP stream?
 

Best Regards