@Cardifftomo 

With your setup I would recommend Deco M9 Plus. It looks similar to M5, but slightly larger. It provides better WiFi signal. I know that for a fact as I run mix of M5 and M9 Plus in my house and tested them both.

For you to gain benefits from M9 Plus, you must replace all three Deco M5 you have with Deco M9 Plus. Replacing just one or two units won't work, you must replace all you have.

If you ready to consider this, note that while M9 Plus should do better, I can't guarantee it will be good enough to meet your expectations. Only practical tests will tell. Get M9 Plus set from the place with good and simple return policy. 

Before you do anything, check where Deco M5 in "Man Cave" is connected to. You can check it in Deco app, under Signal Source for that Deco. Example:

In this example my Satellite Deco connected to Deco I named "Living Room."

If "Man Cave" Deco is connected to Main Deco in the house, try making it connect to "Garage" Deco instead. For that, you will need to download and install firmware 1.6 beta on "Man Cave" Deco, and then you'll have an option to change where it connects. See more information here: [New Firmware] Deco M5 1.6.0

You will only need to update "Man Cave" Deco with firmware 1.6.

After you've done that, check "Man Cave" Deco performance, it might improve.

In case you find "Man Cave" Deco is already connected to "Garage" Deco, replacing M5 with M9 Plus is the way to improve WiFi performance. 

Unless, of course, you could put Ethernet cable from Garage to Man Cave and connect both Satellite Deco with Ethernet cable. That'll improve "Man Cave" Deco WiFi performance to that of "Garage" Deco, without replacing M5s you have.

@Alexandre. Thank you for your reply great help I'll take a look at the m9 plus system later on

i was looking at the x60 as says coverage of 70.000sq ft

Cardifftomo wrote

 

i was looking at the x60 as says coverage of 70.000sq ft

@Cardifftomo 

I think they meant 7,000 sq. ft. 

Be skeptical to what WiFi mesh manufacturers claim for coverage. It is like an MPG sticker on a new car: good to compare efficiency of cars relative to each other, but in real life your mileage (per gallon) for a specific car may vary.

Bottom line: you will still need three Deco units. Consider this when making price related decisions.

From what you draw I can conclude that biggest challenge in your Deco mesh is not WiFi signal from Satellite Deco unit to WiFi devices, but connectivity between Deco units. This is where Deco mesh must be losing speed.

That is the reason I recommended M9 Plus: not only it has stronger WiFi signal than M5, but it also has separate WiFi band for backhaul, for connectivity between Deco units. That is what should improve your Deco mesh performance. Deco M9 Plus is what they call tri-band (two WiFi bands for devices, extra band for connectivity between units).

Deco X60 is not tri-band. Switching to it shouldn't improve your Deco mesh performance by much. If you want to expand Deco models for consideration, check Deco X68 and Deco X90. Each of them is 3-band.

Deco X68 is not released in all regions yet, and Deco X90 is top of the line Deco model, most expensive. Still, if you can get X90 on sale for the price meeting your budget, go for it - you could not get anything better from TP-Link. 

If X68 is available in your area and by the price similar to M9 Plus - go for X68.

In summary, this what you should consider as Deco M5 replacement: Deco M9 Plus, Deco X68, Deco X90. Any one of those must be bought in 3-unit set.