The easiest idea will be to place your router to the middle of your flat (to the top of right angle of L). You could do that with a LAN cable.
Of course any repeater or extender will help. But I'm not sure if it is legal to use extenders at your place. You know in some countries (e.g...
Refog Employee Monitor
MIPKO Employee monitor
And many others.
Well basicly you could google it. In fact most of them have trial period, so you could try.