@lapsmith 

Just wanted to update, the wifi has been stable so I'm very pleased with the beta version.  The app is still incredibly slow to update (minutes, not seconds) but a small price to pay.  Thanks again.

Tom

  @David-TP  Thankyou so so much. For those thinking of doing this with just an iPhone it worked fine for me with no hiccups.
 

I followed your link. Downloaded the Beta firmware to my default iPhone local files area. Made a new folder called DECO at top level in files and extracted the .bin firmware from the .zip. Made sure my iPhone was well charged.
 

Went to the web interface of my Deco. I had to edit https:// to  http://tplinkdeco.net  as my browser complained about the certificate (DuckDuckGo as I wanted a clean browser with only one connection to my Deco S4; leaving Safari with tabs open to this and various other useful pages). 
 

From the webpage browsed around and all seemed to be working. Went to Advanced -> System -> UpdateFirmware. Browse to my local files area and select the .bin and then watch as it uploaded ;  installed  ;  rebooted ; and works with the extra preferences option line in the DecoApp when viewing the satellite nodes.  
 

Thanks again.