Unless you live a very dynamic lifestyle that requires your calendar to be 24/7 synced, you can just use whatever server software you like, make it listen in LAN only, and have your devices sync when they’re at home.
DecSyncCC and Syncthing is another option.
Maybe use a filesystem and syncthing this way? https://github.com/39aldo39/DecSync
Or you could use the free tier of some PaaS like
https://render.com/docs/free if you prefer a traditional approach. Just remember to make backups, if it’s free it can disappear at any time.
It just works, there’s no “how”. Take one of the devices outside, connect to the internet, done.