Requirements

A synced network needs two things beyond the plugin itself.

A shared database

Every server must connect to the same database, and it has to be one that can be shared: MySQL, MariaDB, or PostgreSQL. SQLite, the default, is a single file on one machine and cannot be shared between servers, so it is for single-server setups only. Point every server at the same database details in config.yml, and see Database for setting each engine up.

Redis

You also need a Redis server, version 6 or newer, that all your servers can reach, with redis.enabled: true and matching redis settings on each. This is what keeps the views live. See Redis.

Do not forget proton.lock. Every server that shares the database must also share the same proton.lock file, or it cannot read the encrypted auction data. Copy it to each server alongside the database details.