@Wiggers 

Hey

Sadly the issue may not be the X20, rather the device that is connected to it.  Let to try to explain

The X20 has quite a powerful antenna, it can likely send a signal 40m and you will see a strong signal at the far end if you use an app like WiFi Analyser (Android).  However to be able to work correctly, this is a two way conversation.

The Phone / Laptop whatever that is down the garden wont have the same strength of antenna to send the 40m back to the x20, therefore it appears as an issue.   Sadly devices dont have the same power as an AP does, therefore the communication is just one way and fails. 

You could test this with a laptop and decent external antenna (USB dongle with attached aerial) and see if that works, then you know its not the AP.   OR  something like WiFi Analyser for Android will tell you the signal in -db at the far end, I would start with that before looking into other things.   Post the -db here and we know then what the plan is. 

If its -70 db then its dead.. you need another AP to repeat it.   Else if its -50 to -60 then the client device cant reply as mentioned.

Start with that and see how you get on, feel free to reply.

  @Philbert Surely the sensitivity of the receiver in the X20 plays a significant role? My phone detects the AP and initiates a connection, but it gets dropped almost immediately.

  @Wiggers 

It sadly doesnt work that way, yes the bigger and better the receiving antenna is the better.. however if the signal coming back is weak its likely experienced interference and corruption on the way back, therefore it may be un-readable and needs retried.

What you describe where it connects then drops, thats symptomatic of a client not being able to respond to the AP.    The initial connection process when connecting to WiFi is pretty low overhead, therefore few devices fail to connect.  Its only when actual data is sent a second or two later that it fails and drops connection.

If I was in your scenario, I would get WiFi Analyser on an Android device and see what signal you are getting at the far end, this will give you some idea of the best course of action.   Also dont use Apple devices for this, they generate false results when testing WiFi signals (its a long story)...

  @Philbert I'm not convinced by this, having had some experience with military comms.

I was looking at the Deco X50-Outdoor, which bizarrely claims a coverage of 2500sqft. Is that 25x100ft, or 50x50ft or a circle of radius 28ft?

  @Wiggers 

If its comms as in RF based, its very different from WiFi.  When you add massive amounts of OFDM across the wavelength it becomes hugely susceptible to intereference and therefore corruption, if a signal is weak then this is greatly exasperated.    RF does have some OFDM solutions but at much lower levels, or sometimes COFDM

Either way, you need to test this.   Just telling you what I have seen a hundred times, sounds like your issue.

In answer to your question on range.. take this with a MASSIVE pinch of salt.   All manufacturers claim what is possible in theoretical maximums.   The 2500sqft they claim is in a donut type radius without the effect of a fresnel zone, so basically its pure maths.   In reality its going to be around 100m range in a 360 horozonal to the AP provided there is clear line of sight.   100m is usually what most people would choose for a directional Point to Point link.    When using omni directional antenna (like inside the deco) this is reduced considerably.

However as mentioned earlier, just cause you can send to 100m... doesnt mean the client can send back at 100m.  Laptops will perform better than phones / tables due to the increased power and antenna size.   Until you do some testing its just purely speculative. 

  @Philbert I'm actually getting around -80dB 3/4 of the way down the garden and reasonable throughput.