I would use snackbar in order to show small messages about the status of the complaint, however it should not contains to many information.
Snackbars provide lightweight feedback about an operation in a small popup at the base of the screen on mobile and at the lower left on desktop. They are above all over elements on screen, including the FAB.
You can also add a link to a page describing in details your complain information.
For more information about best practice about snackbar see Material design snackbar page
If you need a more prominent display, Material Design suggests to use the Banner component
A banner displays an important, succinct message, and provides actions for users to address (or dismiss the banner). It requires a user action to be dismissed.
Banners should be displayed at the top of the screen, below a top app bar. They are persistent and nonmodal, allowing the user to either ignore them or interact with them at any time.