The Talk (Spreed) server itself has to be installed on the Nextcloud server. It’s an app which can be installed via the Nextcloud App Store like any other app. Of course for installing it that way you have to be online. If you want to install it while your server is offline you could download the tar.gz from Github and extract it manually to your /nextcloud/apps/ folder… Maybe this topic is of any help: Manually downloading (installing) apps - #15 by hcgrove
As for the STUN/TURN server (coturn), there are plenty of topics and guides in this forum and in the web about how to install and configure it. It can either be installed on the same server as Nextcloud or in a separate VM. Coturn is a Linux package which is included in the repositories of all major Linux distributions. You just have to find a way to install it while you’re server is offline.
There is also an optional Signaling server aka Talk High Performance Backend which you might want to install in a seperate VM. I never did that myself. But you should be able to find instructions on the Internet on how to install and configure it by using the search engine of your choice. The installation process may require an internet connection. However, for day to day operation in your LAN, no Internet connection should be necessary.