I'm living inside a study union for the next few month. the organisation has its own network and gateways where the students are connected to the internet. problem: I don't have access to the routers but want to install my sftp-server. is there a possibility to access from outside the LAN to my server via sftp?
Only if you tunnel it from the inside to a server you can access from the internet first. You have to forward a port on a remote, publicly accessible machine to the port on your computer on which the
sftp-server is listening. Basically you need to run on your server something like:
ssh -R public_port:localhost:internal_sftp_port public.server
ssh(1) man page and also OpenVPN port forwarding without router forwarding?