Actually in our company we have install a VNC Server on every single computer.
To access them from "outside" i had to add NAT for each internal server to the firewall.
It there existing something like a "Master Server" or "Gateway Server" which i could for example access and it shows me the internal vnc servers which are online and allows me to select to which one i want to connect?
you need the ability to connect to and remotely control each desktop in your company to provide support/assistance to your users
-- You should ask that question. As it stands right now your question is the XY problem. You're trying to shoehorn a solution that probably isn't the right one...