@DAServices 

Would it be possible to have a Ethernet cable from the main Deco to the satallite Deco? Either one long cable or putting a switch inbetween and connecting both to that switch.

Just to test, have you moved the TV Tuner to the main Deco to verify the bottleneck is somewhere at the satallite Deco?

@Tony  Unfortunately, the Main Deco is on the first floor family room addition on a slab. The satellite Deco and the SiliconDust HDHR Scribe Duo is in an upstairs bedroom. They are in a second story bedroom due to; 
#1. The necitity of mounting the antenna on the South facing wall high enough so that the antenna can pickup the Signals coming from the East and the West.
#2. There was already an exterior brick wall penetration by an unused old cable line. The homeowners did not want any more Cat6 cable run on the exterior,than was absolutely necessary. 

Initially I used an AC600 Powerline to extend an Ethernet line to the second floor bedroom. But the AC in the Family room where the main Deco resides, is apparently crossing to a secondary circuit panel or some other issue, as the throughput on that ethernet was less than 6Mpbs. And the ISP router was unable to reach the second floor bedroom with a signal better than -63Dbm.

So now enters the Deco M5.  Initially the signal from the main Deco to the second floor was in the -50Dbm range. Setting up the satellite Deco M5 in the second floor bedroom and attaching the SD HDHR tuner to the Ethernet worked beautifully. 
The Wi-Fi signals on the first floor and second floor were all better than -50Dbm.

The streaming Live TV signal was great on the first floor family room Roku (less than one foot from the main Deco. The streaming also worked flawlessly on Windows Notebook also on the first floor. It also worked beautifully on a Windows 10 PC in the next second floor bedroom.  
All worked for the better part of three weeks.  Then The homeowners purchased a second Roku Stick for the TV in the second floor bedroom which is less thasn 2ft from the Satellite Deco. That Roku could stream Amazon and Netflix, When the SD HD app was used the screen and menu came up but no channels could be selected and viewed. 
All other devices were still able to launch and stream both standard and high definition Live TV. 

I swapped the Rokus and then both of then became unable to stream live TV ! 
After an hour or so they both began working again.  

SiliconDust's response was that they do not recommend Mesh as they have had troubles with Google and Eero.  I tried to explain that it was working for weeks before it stopped working, but  they blamed it on the Mesh backhaul not being able to maintain an 80Mbps stream.  They said that SD TV needed 10Mbps per stream and HD TV needed 14Mbps per stream and 4K needed 20Mbps per stream.

I countered that 80Mbps would only be needed if there were two 4K streams and there never is. There is never more than two HD streams which would mean that the backhaul would only need 30Mbps sustained. 

@DAServices 

I agree as well with the bandwidth demands not needing to be as high. Signals do fluctuate and will affect speed. When you notice connections slowing down run a speed test on your phone to see if the Deco speed is dropping lower then what it should be at.

The Decos do have an AP mode, the aim would be to try and cut any overhead and see if speed is increased, but the first Deco would need to be wired to a router, and not just a modem. The mesh and wired backhaul would still work normally.