@terziyski 

This unfortunately didn't resolve my issues but thank you anyway . It was a good insight to other potential issues. 
My mower will connect straight away to my main router and therefore I assume those settings are fine. 
ive also connected it to my phone via a hotspot and that also worked straight away. 
 

Its obviously something in the Deco settings that's preventing my device from picking it up . 
Either that or the device isn't compatible with the deco for whatever reason . I hope its not the latter as the main reason for the deco was to extend WiFi into garden for the mower🙈.

  @terziyski 

I have tried in Both AP and Router modes and even turned off the WiFi on my Talk Talk router in case it was confusing the mower . 

In the app the only wireless settings I have are :

security - set to WPA / WPA2 as recommended on mower.

band - set to 2.4ghz 

hide network- untoggled . 
 

is there any more options / settings I am missing ?

  @terziyski 

I have tried everything listed to no avail .

i have sent yard force an email checking wether it is compatible with the Deco as it still connects immediately to my old WiFi so something isn't right . 
 

I will update with a solution hopefully 🤞. 
 

Thanks for your help .