How to extend internet coverage on this apt?

I'm a network novice, so please bear with me.

I want to extend inteet coverage throughout the apartment, and I have no idea how to do it.

Our apartment looks more or less like the image below. The issue is that the concrete walls are thick at about 1', and there's a block of concrete (marked concrete) that's about 6x2 feet. The red indicates the electric outlets.

My question: how can I extend the inteet from the modem to those rooms in the house? I was considering wireless extenders, but I have no idea how many I should buy given this layout, or if there are better alteatives.

Being the begier that I am, What I did was coect a cable from the modem to the router. Then the PC that's the farthest away would be in the range, but just barely. The inteet coection in that PC is very weak.

Thanks.