Question：

I have my industrial router USR-G806 connected to the internet using a 4G sim. Because the IP is not static, I need a VPN connection to access the router :
USR-G806 / LAN = 192.168.1.0
USR-G806 PPTP interface = 192.168.3.11
PPTP Server = Mikrotik router
  
The problems :
1 - I can only access the USR-G806 web interface if I am connecting to the same VPN server using PPTP
If I make a port forward on the Mikrotik router <public ip>:8311 -> 192.168.3.11:80 it will not work
2 - I have a web camera connected to the LAN port of the USR-G806. How can I access the RTSP stream using the PPTP interface and a port forward like I explained above?

Answer:

After your VPN is successfully established, you need to make a port forwarding from the USR-G806.
Access from the WAN port of the industrial router USR-G806 to the LAN port.
Also, change all of the Basic Settings options in the firewall to Accept. 

Please refer to the webpage:

https://www.usriot.com/news/port-forwarding-of-industrial-router-usr-g806.html