Locally you can turn cloud security off ( Settings / Configurations and then edit tab Server). In the cloud it depends on which Mendix cloud you are. Mendix V4 should work out of the box and for the V3 cloud you need to file a support ticket to change the security policy.
Regards,
Ronald
[EDIT]
I am no Oracle specialist but this website might give a clue: https://chartio.com/resources/tutorials/how-to-fix-ora-12505-tns-listener-does-not-currently-know-of-sid-given-in-connect-descriptor/