Debian 13 as a dual WAN router

What happened

I depend on Internet access. It should always be fast and it should always work. For a few years a dual WAN Cisco router took care of that, with a Ziggo cable line and a KPN fiber line. I love Debian, so when I replaced it I chose an N100 mini PC with six gigabit ports and an NVMe disk, and turned it into a router with Debian 13. This post describes the setup and the things that went wrong along the way.

Summary assembled by rule from the sources below

Sources