I’ve fixed it!
After taking a closer look at the blog post you shared, I added ssl=true to the comma-seperated connection string and it finally worked!
So. here’s the final connection string that worked:
"mongodb://john:PASSWORD@gdelt-shard-00-00.n1mbb.mongodb.net:27017,gdelt-shard-00-01.n1mbb.mongodb.net:27017,gdelt-shard-00-02.n1mbb.mongodb.net:27017/DATABASE?ssl=true&authSource=admin&replicaSet=atlas-7o9d3y-shard-0"
And the link to the doc explaining the ssl part of mongoDB URI for anyone that might come across this later: