@NickEnglish 

It is possible MAC address randomization on iPhone confuses Deco mesh somehow, or your sky router runs out of IP addresses because of MAC address randomization.

To check that theory, turn off MAC address randomization on problematic iPhone for Deco mesh SSID. Instructions at the bottom of this post.

If that does not fix the issue for that iPhone, in addition to that configure static IP address (IP address reservation) for iPhone in sky router. 

Apple iOS 14 & Later:

• Ensure your device is connected to a Wi-Fi Network
• Open the "Settings app", then tap "Wi-Fi"
• Tap the "information button" next to your Deco network 
• Tap on the "Use private Address" toggle to turn it off
• After, a privacy warning will pop-up

  @Alexandre. Thanks. Changing the privacy setting did not seem to make a difference. Will look at the sky router

  @Buff-man Thanks. Fast roaming and Beam forming are turned off on the TP network are turned off. Smart DHCP is enabled.  Nick

  @David-TP . thanks for your post. One phone is now on 17.0.3, the other still on the list 16 OS. The third iPhone is on 15 something and has no problem. The Decos' firmware is updated.

Overnight my phone kept the signal but this morning it is still the same....stays "connected" when I look at WiFi settings but phone loses the WiFi signal. This is whilst in the same place, not moving around Then loses the 'connection" in the settings. It does not find "my network" automatically. The fast roaming and beam forming technology on the Deco are turned off.

I will turn off private WiFi address on the phone setting to see if that does anything. As you can imagine, this is very frustrating! Nick