Ok, I think I have some kind of answer to that!
On Mongo Atlas one has to whitelist their IP address for letting connect.
It is a 'hardcoded' setting, but my public IP is changing from time to time. ( I am not sure why, since i am not an expert of the topic).
So when I am whitelisting my IP, the problem seems to be solved.
Only It would be nice not having to update the IP manually everytime it changes. We do not know when it changes, so this can become uncontrolled. The other option is letting connect from every IP, which I am not really comfortable doing yet.