Over how many floors can powerline work?
As already written, the power grid usually consists of 3 phases. In Germany it is. But also typically there is every phase availabe in each apartment, mostly split into different rooms. There are so-called phase couplers which connect different phases at hf-side.

The problem is more the length of the line and the interference on top of it. And there are many of them: power supplies, photovoltaic systems, etc. You cannot make a statement like this, you should simply try it.

An alternative for you would be WiFi. You could install an antenna on the balcony. The signal should be sufficiently strong to receive over the free line of sight.