@Yanaok 

You can't bring WiFi through the well made concrete walls, that is limitation of wireless technology. What you may be able to do is bring wired Internet link to that room, and attach Deco M5 to it.

Ideally, it should be Ethernet cable, if you can lay it from the house into that room. If you can't, but room has electricity coming by the wires from the rest of the house, try powerline adapters. They only work well if they are on the same circuit breaker.