If you provide correct license key then the only way it may happen is that you actually use
Demo dlls. The overlay shows reminder with the word
Trial if it is a demo version, demo version can not be activated with any key.
You should use full registered version of the dll (available from your licensing dashboard). Full version will show a warning overlay with unregistered word if incorrect license key is provided.
Please note again that dll from demo version (free trial version from our website or from NuGet) can not be activated at all, and will always display overlay even with a license key.
If you still having issues, please send us the screenshot of the overlay message.