So in 2013 I bought the WPA220 AV500 KIT (Powerline and Extender) - The only setting I ever changed was to turn off Power Saving Mode.

They worked great for almost 10 years. (They were on the latest firmware from 2016)

Anyhow I thought I'd upgrade to the WPA220 KIT V5 as newer device, newer technology (better performance).

Anyhow I set them up and put them in exactly the same locations as my old devices.

The Extender for the V5 is in our 2nd Story room (exactly where the V1 Extender use to be) and Ethernet and WiFi for my Macbook M2 work fine.

However our iPhone 14's are getting terrible WiFi speeds from the Extender?

It only happens with mobile devices.

Now I changed every setting possible, WiFi channels, Frequencies, I even changed the location but still terrible speeds on mobile devices..

I got my V1's, reset them and set them up again and BAM everything is flawless again.

I am wondering what went wrong with the V5's, does anyone have any experience of this? 

The situations were absolutely like-for-like and I even updated the V5 to the latest firmware.