How does app is tested

Is the first step (login) handled automatically? Are all models tested simultaneously, and if one fails, does that cause all of them to be rejected? My app was recently rejected with the reason ‘invalid credentials.’ All models were rejected at once, but I’m confident that the credentials are correct. The previous version was approved with the same login details, and there were no issues. I need the new version of the app to be available as soon as possible. I’ve submitted a support ticket, but it usually takes a few days to get a response, and I can’t wait that long. If the rejection was due to an error with the first tested model, could I resubmit it using the others? Does anyone know how the testing process works?