I can reach the admin tab from my lan, but the connectifity test fails, and I can't reach the server from outside. Already tried to change the TLS Settings but nothing changed. What kind of logs do you need?
Did you get the client install OK? Did you forward ports UDP 1194 and TCP 943 from your router to your OMV??? And open these in the firewall on your OMV if you are using it?
If you follow exactly the instructions on the plugin it should work. The connectivity test will fail. That is OK if it fails. With the setup we use it will not pass all the tests for the connectivity test.
Are you testing with a computer or mobile device that is outside of your LAN ???
A lot of routers today will not let you access your LAN with a WAN IP when you are in the LAN.... It is a security measure. Allowing this is call NAT Reflection. Your firewall on the router should be working. That is why you have to open/forward ports.