Good news: the 2-3 day delivery was more like 24 hours. New modem!
Good news: successfully found out the account information for the internet.
Bad news: still not working. Flatmate's just gone to work and I cant find whether we have another wall jack to test. The router has power and it's hosting the wifi network, it just won't connect to the internet. It's not a modem problem or a cable problem, I switched all those out for new.

So I'll wait until flatmate's home again and see if we have another jack hiding behind some heavy furniture or something. Otherwise we might need some actual tech support because I'm about to hit my limit of helpfulness.

Bad news: flatmate's sister doesn't know of another jack either.
Bad news: troubleshooting at a neighbour's house suggests the jack is not the problem, and also that I shouldn't be trusted to cross the road at night. (The streets here are not always generous with the lighting...)

Good news: apparently this error message means the modem is configured for vdsl instead of adsl
Bad news: configuring for adsl doesn’t actually fix it.