So, I'm going to try my best to write everything that we've tried, and I would really appreciate it if someone could lead me through the process that will succeed finally.
The first thing we did was follow the tutorial from Youtube, installed everything but that work just while we are connected on the same network (while we were working physically together in my apartment). After that, we tried through Hamachi, and the colleague managed to connect, but a minute after he gets thrown into offline mode.
The last thing I tried was making a Google Cloud Windows server. Opened the ports through GCP, created a static IP address, and after DAYS of fiddling, I got info that since the last year EULA change, BIMCloud Basic does not work from servers (kudos to Croatian and Danish distributors who were helping me last two weeks to try and make this work).
I doubt that Graphisoft made this impossible for students and there must be a way to make this work. Please help me! Easter break is coming we need to set it up...