I want to sent large amounts of data over this Server.
So would STUN work?
I mean it would be pretty good if I could actually use the STUN from google.
Google STUN servers are for streaming audio and video, they are for WebRTC, so yes you can stream or send large amount of data.
It's immportant to know that the Windows-Server-Service will be behind different corporate firewalls I can't control (And apparantly even most admins suck at adminstrating those systems).
The Mobile Apps know the WAN-
IP of the Windows-Server.
But there are many Mobile Apps and many different organizations with their own Windows-Server.
And the data should allways be sent over the STUN Server . No real peer2peer conection. no interactive exchange of adresses.
If by WAN-
IP means Internet accessible
IP then you don't even need STUN or any sort of tunneling/forwarding, STUN and TURN and other protocol used for either bypass the NAT and firewall restriction or to provide privacy (hide
IP's from each others) in case peer-peer connection.
I highly suggest to check
https://www.esegece.com/websockets and may be test drive it, as it provide all the security you need for your data, while LakeOfSoft will need experience to add such secured connection by hand.