Nextcloud talk for clients within restrictive firewalls

I have a CentOS 8 server running nextcloud / talk with nginx/1.14.1 and php/7.2.11.
Is there anything I can do, to allow video/screen sharing for clients behind restrictive firewalls?
E.g., within my universities network, all incoming UDP packages are dropped…

I have installed coturn, but image-streams remain blacked out. This is probably not surprising, as the turnserver is outside the clients firewall as well…

Is there a way? Thanks for ideas :slight_smile: