Response from Gosia:
Dear Susie & Jan,
many thanks for your stay. I really appreciate your suggestion regarding the wifi connection. Honestly, I didn't even know about something like wifi extender before you let me know. I bought it and put in the ground floor apartment. ( I was even able to make it work ;). Now the wifi signal is perfect.
Many thanks one more time for this suggestion.
All the best,
Gosia & Mario