Outdoor units are pretty flexible in the sorts of places they can be mounted and the insulated refrigerant lines can be run to the interior of buildings fairly easily. Don't just assume a solution can't work until an expert says no.

And even if you can't hook into your existing radiators wall mounted mini splits don't require extensive interior changes to function (plus you get cooling as well)

I do have a mini-split AC which can also provide heat. It is OK for autumn and spring but it is a no match for a radiator heating when it's really cold outside in the winter. Yes, of course I can run the lines through the whole apartment but this is a much costlier proposition.