@IanSutcliffe 

The only real way to fix a dead spot is to either move the current nodes around to try and cover that area, or add an additional node to your setup to extend the range.

Deco nodes can be mixed so you can add any additional node to the current setup, should you require more.   

The best place to start is seeing if you can move one of your existing Deco a bit closer to the black spot and see if that helps

  @Philbert 

Thank you. 
 

I now have a M5

  - in our living room. (Strong signal) in cabinet with router 2' above floor (1)
  - in Hall way outside office (strong signal in hall and office) 1' above floor  (2) 

  - in our hall closet opposite our master bedroom (strong signal in hall) 6' above floor (3)

  - in our 10 x12 master bedroom 18" above floor (4)

      Good signal on left side of room

      Good signal in on suite bathroom 

      Very poor or no signal on the right side of the bed. 
 

We have 4 M5s in a very small 840 Sq ft apartment. 
Per your suggestion, I have tried different positions n the room with no effect. 

The firmware is up to date. 
 

In our old and much larger concrete home we used 3 M5s with excellent results. 

Let me know if you have any more ideas. 
 

thanks again 

  @IanSutcliffe 

Have you considered the possibility that your master bedroom Deco unit may be defective, with broken internal antennas, and not properly broadcasting WiFi signal?

I have Deco M5 at my house. I measured signal strength from it at different distances, with direct unobstructed view between smartphone and Deco M5. Here are results:

1  ft.: -28dBm

12ft.: -45dBm

25ft.: -55dBm

Even at 25 ft. distance, with no obstacles between them the signal is good. See if swapping master bedroom Deco with different Satellite Deco will improve signal at master bedroom.