RBK852 poor coverage in a single room

Hello

I had an RBK40 system which died.  I replaced this with an RBK852 system and have the Router and Satellite in the same locations.  I now struggle with poor reception in a single room for both 2.4 and 5GHz network (80-90db signal where as rest of house is well 70-50db)  I am in a congested space from other networks and use Auto for 2.4GHz and 36 for 5GHz. 2.4GHz in this room is better than the 5GHz signal

I have a fairly small apartment of 135sq meters.  Its oblong shaped and the hallwalls are in a U shape.  The router is at the midpoint of the U on one side and the end of the U on the other.  (see pic)

The backhaul status of this good and is on 5gHz.  There are approx three 10-15cm concrete walls between Router and Sat.

If I move the Sat to the Poor reception area then it works fine and remains at good backhaul and 5GHz but now I have poor reception in the room it came from.  

I am on latest firmware.

Would another Sat perhaps be a solution at the red star? My apartment is not that big !  I can't currently move the Router but plan to try once I have a longer Cat5 towards the red star.  I cannot understand why my old RBK40 system worked fine and the new does not.  Also why does back haul remain fine but not client facing.

Thanks