Hey. You can try the custom AsusRouter integration I am developing. It has the functionality to raise HA events when new devices are connected, reporting all the main info about the new device - MAC, hostname, IP and more. It can be easily used for automations and so on. More details on the event can be found in the integration docs.
There is also a user-friendly how-to start guide. The latest discussions on the features and how to use them can be found here: [Custom component] AsusRouter integration - Share your Projects! / Custom Integrations - Home Assistant Community (home-assistant.io)