As far as I know Talk used to be named Spreed. Before Talk I was running a separate Spreed instance. For any Webrtc solution like Talk or Spreed to work you need STUN or TURN and signaling. I run Coturn for instance to connect clients behind NAT on a separate server.
For some reason getting a signaling service working is hard right now. Nextcloud does offer a solution, as far as I can tell it’s a secret what they are using. It would be great if they release this proprietary information at some point.