@CliffH Hello,
Please confirm your range extender is on the latest firmware:
https://www.tp-link.com/support/download/re220/#Firmware
May I know how often iphone drops connection from range extender,what is the network activty on the iphone at time, like streaming, browse website etc.
Does other devices in the same location work fine with the range extender? Does iphone work stable when connected to router?
Here is a detailed troubleshooting guide to for RE unstable issue, please give it a try and feel free to let us know if helps:
https://community.tp-link.com/en/home/forum/topic/272398