Couldn't connect with example stack.yml from documentation

[LeeviHalme]

I am simply running docker-compose up -d using the compose file found on the docker page of mongo (https://hub.docker.com/_/mongo/) under the title "... via docker stack deploy or docker-compose"

docker-compose.yml:

# Use root/example as user/password credentials
version: '3.1'

services:

  mongo:
    image: mongo
    restart: always
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: example

  mongo-express:
    image: mongo-express
    restart: always
    ports:
      - 8081:8081
    environment:
      ME_CONFIG_MONGODB_ADMINUSERNAME: root
      ME_CONFIG_MONGODB_ADMINPASSWORD: example
      ME_CONFIG_MONGODB_URL: mongodb://root:example@mongo:27017/

Output of running docker-compose logs

mongo-express_1  | Welcome to mongo-express
mongo-express_1  | ------------------------
mongo-express_1  | 
mongo-express_1  | 
mongo-express_1  | (node:7) [MONGODB DRIVER] Warning: Current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
mongo-express_1  | Could not connect to database using connectionString: mongodb://root:example@mongo:27017/"
mongo-express_1  | (node:7) UnhandledPromiseRejectionWarning: MongoNetworkError: failed to connect to server [mongo:27017] on first connect [MongoNetworkTimeoutError: connection timed out
mongo-express_1  |     at connectionFailureError (/node_modules/mongodb/lib/core/connection/connect.js:362:14)
mongo-express_1  |     at Socket. (/node_modules/mongodb/lib/core/connection/connect.js:330:16)
mongo-express_1  |     at Object.onceWrapper (events.js:420:28)
mongo-express_1  |     at Socket.emit (events.js:314:20)
mongo-express_1  |     at Socket._onTimeout (net.js:483:8)
mongo-express_1  |     at listOnTimeout (internal/timers.js:554:17)
mongo-express_1  |     at processTimers (internal/timers.js:497:7)]
mongo-express_1  |     at Pool. (/node_modules/mongodb/lib/core/topologies/server.js:441:11)
mongo-express_1  |     at Pool.emit (events.js:314:20)
mongo-express_1  |     at /node_modules/mongodb/lib/core/connection/pool.js:564:14
mongo-express_1  |     at /node_modules/mongodb/lib/core/connection/pool.js:1000:11
mongo-express_1  |     at /node_modules/mongodb/lib/core/connection/connect.js:32:7
mongo-express_1  |     at callback (/node_modules/mongodb/lib/core/connection/connect.js:300:5)
mongo-express_1  |     at Socket. (/node_modules/mongodb/lib/core/connection/connect.js:330:7)
mongo-express_1  |     at Object.onceWrapper (events.js:420:28)
mongo-express_1  |     at Socket.emit (events.js:314:20)
mongo-express_1  |     at Socket._onTimeout (net.js:483:8)
mongo-express_1  |     at listOnTimeout (internal/timers.js:554:17)
mongo-express_1  |     at processTimers (internal/timers.js:497:7)

• Relating (basically the same) issue is #506 . That was closed and forums were recommended.
• Relating forum post: https://forums.docker.com/t/docker-compose-service-name-problem-using-mongo-official-image/126235

Waiting for MongoDB to start up wasn't the issue. It was still failing to connect after 10 minutes

Any ideas how to fix this? Host machine is running Ubuntu.