I have the same issue since Monday 20 april 2020 the connection is suddenly dropping.
Ziggo gave a replacement modem (in stead of the old balck UPC modem with only 2,4GHz, n WiFi antenna, I now have a standing white Ziggo modem (with ac WiFi antenna).
But last Tuesday it happend again. That is not nice when you have to work from home over VPN.
It seems to happen at 7 am, around noon, around 9-10 pm.
Last Tuesday it was every 15 minutes.
Can Ziggo please take a look what happened since 20 april 2020.
I also heared KPN customers complaining on Monday 20 april 2020