Hello @Will_Jones
If iphone fails to connect to extender Wi-Fi, you could try some suggestions in thread here:
1. Confirm Range extender has the latest firmware (which your RE already got)
RE220 firmware: https://www.tp-link.com/support/download/re220/v3/#Firmware
2. make sure iPhone could connect to Router WiFi by manually put in router WiFi password (router and extender use the same wifi password after RE set up)
- remove Router Wi-Fi profile on iPhone and reconnect router WiFi, will iphone take the password
How to remove existing wireless network profile on Android&iPhone
If still does not work, may I know 1) model & IOS version of your iPhone 2) a photo of the error message on iphone 3) the model of router 4) Led status on RE, thank you