The issue is that teiid mongodb connector expects connection parameters based on the " Standard Connection String Format" instead of the "DNS Seedlist Connection Format", as defined in https://docs.mongodb.com/manual/reference/connection-string
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"In addition to the standard connection format, MongoDB supports a DNS-constructed seed list. Using DNS to construct the available servers list allows more flexibility of deployment and the ability to change the servers in rotation without reconfiguring clients."_

The following is a typical connection string for a DNS seedlist connection string:

mongodb+srv://server.example.com/

and:

• a port number is not needed
• a different prefix is used, instead of mongodb:// => mongodb+srv://

I have a "free tier" cluster instance on cloud.mongodb,com (aka Atlas), which is limited in size and resources but should be functionally identical to a full sized paid cluster on the same platform, and it does not expire. A free tier can be activated here
https://www.mongodb.com/cloud/atlas#atlas-form-container

I tried with both the mongodb command line client and with the GUI tool Compass (https://www.mongodb.com/products/compass) which has a community edition

and this different syntax works perfectly, while it is not supported by the current teiid connector, it seems

there is a way to use the "old" syntax, prefixed by "mongodb:// " with server and port, to those Atlas clusters, but it is cumbersome, as it needs the user to specify more parameters, as "replicaSet" and "readPreference", at least. See https://docs.mongodb.com/manual/reference/connection-string

With those additional settings I was able to connect with the Compass GUI tool, but not with the latest command line client (4.2), and anyway the teiid connector seems to not support those additional parameters,, I tried to add them as DS properties but always failed