Try simple installation using leaky coax (radiating coax) cable.
Put some antenna at the end and set it in place where signal is ok.
Then use standard coax - next in Your apartment connect few meters of
leaky coax hidden on the floor for example.
This could help as it's common solution in buildings.
If You doesn't have any possibility to buy leaky coax /it costs
quite - about 4 Euro per meter/ then on the other side of coax from
antenna - connect another antenna - just like the first one.
Those solutions are quite cheap to test.
Good luck.


Regards
Andrzej Rusiecki