thank you all. i've almost resigned myself to the fact that it doesn't work with simple gui settings. i also don't know enough to tinker with config files on my own. i asked my colleague again about his settings on synology. he redirects port 443 to the default port 1194 via his home router. So he uses the router-nat, is reachable from outside via 443 and simply routes to the VPN instance.
there is probably no comparable NAT function in cloudron, is there?
while searching the internet i found the "haproxy" in docker-hub. maybe such a container (app) could transparently redirect from a host with port 443 to an internal ip with port 7494. but this is probably going too far and i don't want to overuse your help.