the problem is that I can't connect to VPN through the internet connection, but it works on the local network.

I use no-ip.com service to connect to the first router, then It should allow me to connect to VPN on router 2 to access files from a computer on router 1 network.

[![enter image description here][1]][1]

All PCs from router 1 and 2 networks have no problem with connecting to VPN.

The first router is D-Link DWR-921 and it is 192.168.20.1 IP

The second router is ASUS RT-AC51U and it is 192.168.21.1 IP

For some reason, the first router can't access the second network, but second can access first.

  [1]: https://i.sstatic.net/Qk8nr.png

EDIT: 

Computer I want to connect to ip is: 192.168.20.3 and mask is 192.168.20.1, for now I've abandoned no-ip and using external ip to try and connect to vpn.