> our landlord refuses to open the door to the cellar

The simple solution would be to make this illegal, i.e. require landlords to allow at least two competing wired ISPs to connect each household.

No need to make them pay for it; I suspect it would be more than enough to end their very lucrative arrangement of somehow rewarding exclusivity. (I don't have any evidence that landlords are getting paid for it by Vodafone directly, but I highly doubt that there's any above board reason for the status quo.)