Hi!
Here is the solution of this problem:
https://habr.com/ru/articles/272461/
You have to add route to your server for VPN traffic
sudo /sbin/route add -host 192.168.0.20 -interface ppp0
192.168.0.20 - server IP
ppp0 - VPN interface (ifconfig may helps to see which one exactly )
Have a nice day!