Network isolation prevents Traefik from reaching container backends

When Traefik is configured with --providers.docker.network set to a specific network (e.g., 'edge'), it can only route to containers attached to that network. Containers on different Docker networks are unreachable even if routers exist and match correctly. Symptoms: router exists in API, rule matches, but Traefik cannot connect to upstream, often resulting in 502 Bad Gateway or service showing no available backends.

Check Traefik's configured network with 'docker inspect traefik --format {{json .Config.Cmd}}' for --providers.docker.network flag. Inspect target container networks with 'docker inspect <container> --format {{json .NetworkSettings.Networks}}'. Ensure container is attached to the same network Traefik uses. Add container to network with 'docker network connect <network> <container>' or update docker-compose.yml to include network. Verify connectivity by testing from Traefik container to backend IP:port.