(Newbie) Help for wi-fi setup of a big house + external parts

[mateta]

Hello,

I am sorry if this question is trivial, but I have tried to look into other topics/articles and I did not understand much, hope someone can help me!

I live in a house on 3 floors which has a quite wide external part. On top of this there is another 2 floors house attached to mine and I would like to share my connection with this other side.
The modem connected to the line is situated on the ground floor (fastweb an italian provider requires it to be plugged to the main plug).

With this kind of setup I can't even manage to reach the 3rd floor of the house and any of the other apartments and external parts.

I was thinking that a solutions could have been to get an ASUS RT-N66U and plug it to the modem either through powerline or ethernet cable and put in in the middle floor to improve the reception within the house, but I am not sure this would work and I think it would not solve the problem with the other side of the house and the external parts.

Can anyone help me?

Thank you!

P.S. Realized I have posted in the wrong section, admin can you please move the post to the relevant section? Thanks!