To connect to MongoDB Atlas, the port 27017 must be allowed from the network. Is it possible to double check with your provider that this port is allowed outgoing traffic?
This is totally under your control in the Atlas IP whitelist setting. Please see Configure IP Access List Entries on how to do this.
However I find it curious that you can connect from a fixed network successfully but not from your cellular network. Perhaps you can try connecting using a VPN while using your cellular connection and see if using VPN enables you to connect?
Best regards
Kevin