@Picket 

To stream multiple cameras remotely, when you do port forwarding for multiple cameras, and you should define different External ports for camera, but the Internal ports are always  2020/554, e.g.

1st Camera:  External port 2020 & 554 ;   Internal port: 2020 & 554 

2nd Camera: External port 2021 & 555  ; Internal port : 2020&554 

3rd Camera: External port 2022 & 556 ; internal port :2020 & 554

• ONVIF Service Port: 2020
• RTSP Service Port: 554

• RTSP URL : rtsp:// camera IP Address:554/stream1 
• ONVIF URL: http://camera IP Address:2020/onvif/device_service
• Note for remote control, change camera IP to router’s WAN IP or public IP

If you only use RTSP, only need to open port 554 for rtsp, to watch live feed of second camera, use url rtsp:// public ip:555/stream1 

Feel free to let us know what software do you use and let  us if this helps.