There are often times when the Docker Swarm Overlay network fails to function, with containers on one host being unable to communicate with containers on other hosts, even though they are on the same overlay network.
This is generally caused due to the required ports needed for the overlay network being blocked by the host firewall.
On every host in the cluster, you must allow ports * TCP and UDP port 7946 for communication among nodes and * UDP port 4789 for overlay network traffic.
The default position of most host OS’s is to block all ports, so you need to use UFW or IPTables to ensure these ports are open.