See also my post on https://forums.elderscrollsonline.com/en/discussion/636730/booted-from-the-server/p4

In short, people with the highest plan (1 Gigabit) currently have a modem with faulty firmware, which causes connection
issues with many game servers, including Elder Scrolls Online.

The ISP now has announced that in 7 days they will be upgrading the firmware that fixes these issues.

In the mean time you can still ask for a manual upgrade at
https://community.ziggo.nl/t5/Internet/Internetsessies-verbreken-onverwacht-sinds-Sagemcom-firmware/td-p/1213717