Is it possible to use a server running StartOS as a STUN/TURN server for Using custom WebRTC ICE servers in SimpleX Chat?
No, StartOS runs StartOS packages, i.e. software packaged for iOS. These packages, currently in docker containers, soon to be in more linux native containers, do not currently allow for such advanced networking (but will allow for more advanced networking soon)
You can and you are free to hack StartOS to do anything you want above the container level, but no, so I can say it’s “possible”, but not without work.