In Sandbox, the MxAdmin account does not work.
When you reset the password of MxAdmin in your local modeler, you are only changing the password in your local build not in Cloud. Using MxAdmin is only possible in Paid cloud version.
https://docs.mendix.com/refguide/administrator
However, what you can do is, add a Microflow which creates Admin user in your application and link this Microflow to ASU.
This will make sure that you will have a user to login when you start your application. Make sure you skip creation of user if one exist, otherwise, the second time you run your application in Sandbox, it will fail.