How to force a smartphone to do update?
Neither taking out the sim card nor a factory reset helped 

The following was suggested by Nokia itself. But turns out they didn't think to suggest doing both at the same time.
Although it can be far easier - just turn off Mobile Data, then check for updates. If it's not enough, also restart the phone while keeping Mobile Data off. Suddenly you get updates.
I'd chalk it off as a bug that lets the phone provider save bandwidth for itself even though you actually use WiFi for updates.