What does advertise-addr do when joining a swarm (not initialising)

I get what advertise-addr does when initialising a docker swarm (the X.X.X.X below). But what does it do when joining a swarm as a worker with a command like:

docker swarm join --advertise-addr= --token SWMTKN-1-.... X.X.X.X:2377

I was hoping this would change the route the swarm manager used to access the node/worker but I have no idea how I can confirm this. One way is using the traefik load balancer it gives me a new server ip when joining the swarm but when I ping from within the traefik container I get nothing (all other servers work).

The server I am using to join as a worker FYI is behind a CGNAT and is a vpn address used to access it.