I'm sorry to hear about the mobile authentication issues you've run into. If you're using a local (unfederated) account, then the username/password should be the same. Are you using SSO for authentication in your sandbox? If so, please verify if the authentication prompt you're entering the password on is your IdP's SSO login page, or the out of box unfederated login.jspa form.
Please also confirm if you're signing in using the mobile browser or the Android/iOS apps.