the easiest is to use signal/r and setup a hub, all the servers are listening to.
if you want to code yourself, you will need to write a discovery service, that can be used for the server to discover each other. the servers should ping the discovery service, so keep it informed of the server health. you can use a database (or redis) to track the ipaddress of all live servers. this list can be used to broadcast to all live servers.