You can use any provider you want, those are just recommendations.
To use ACID transactions, you will have to create the collections first.
Temporarily change the backend/.env credentials to the connection URL.
On the backend, run:
npm run db:create
Now revert the database credentials to the localhost ones.
Cloud database providers usually start the database blocking all external connections. You must whitelist the IPs you will use.
In case you want to allow all IPs, the pattern is usually 0.0.0.0/0.