Hi,
changing your locally used database will have no effect on the Mendix Cloud. You cannot change the database in use on the cloud on your own. In Addition the Mendix Sandbox/Free App has other restrictions (eg. cannot run scheduled activities) see: https://docs.mendix.com/developerportal/deploy/mendix-cloud-deploy
You have done this through Project > Settings > Configurations > Runtime, correct?
These custom Runtime settings are not supported in the free environment.
https://docs.mendix.com/refguide/custom-settings#4-database-settings