@Gaby_G,

Honestly, no matter what you will have problems with getting a signal to pass through any sort of concrete walls - even to the extent that many concrete homes have to resort to Powerline Deco systems. More units closer to the wall will technically improve your connection as more of the signal will be able to pass through your exterior wall, however you can only improve it so much since the signals are still subject to the same interference. You may simply try moving your nodes so that they are closer to a window or wall with less concrete or load-bearing metal inside.

If you are looking at adding additional nodes, I would recommend one of our more powerful nodes as they are able to broadcast further/deal with more interference.
 

Depending on your home/lot layout, you may look at the PoE-powered Deco X50-Outdoor to get your cameras and doorbell. You could also consider a PoE unit in your hallway to provide a stable signal to your doorbell.