Paracaue VPN connections are detected and blocked by IW4mAdmin which is a server management tool many server owners use on their servers. The reason IW4mAdmin blocks VPN connections is because they can be used for ban evading. Since we do not manage servers ourselves and they are hosted by the community it is not up to us whether a player is allowed to join a specific server.
If you want to play on a server that you cannot join for this reason you'll need to contact the owner of the server somehow and ask to get your IP whitelisted. That or stop using a VPN.