TL;DR I reverse engineered my Arris TG2492LG router / Ziggo Connectbox and made a Python package that hopefully can be used in a presence detection integration. Because I’m short on time I hope this gets some traction here and someone is willing to help with developing the integration.
The Dutch cable operator Ziggo provides their customers with one of two routers they both just call Connectbox. Either you get the Compal CH7465LG or the Arris TG2492LG. The Compal CH7465LG seems to be already supported by the UPC ConnectBox integration for presence detection.
I tried to get my Connectbox (the one from Arris) working with the above mentioned integration, before I realized they’re different and the API didn’t match. I did some reverse engineering on the frontend of my router and made a basic Python project for retrieving the connected (and online) devices:
(This is still a very basic client and the code needs more documentation 
)
I hope that will work as a base for developing a presence detection integration in Home Assistant (when I disable the wifi on my mobile it’s directly marked as offline by the router).
I was planning on learning to develop for Home Assistant and make the actual integration myself but I’m currently really short on time. I would not know when I could start or even have it finished. If more people want this and someone want to develop this earlier that would great! I’m willing, as long as time allows, to help in any small way possible!