@Atn6 Hello, 

Thank you for your detailed post. 

It is recommended to confirm WA854RE is the on the latest firmware : (select hardware version first and download the latest firmware)

https://www.tp-link.com/support/download/tl-wa854re/#Firmware

Try a different device to connect to range extender WiFi and see if it could connect to RE's SSID. 

It is recommended to set up a different SSID for RE from router's SSID.

Turn off  Advanced settings on router like Access Control/ Parental Control/ Band steering like this FAQ: https://www.tp-link.com/en/support/faq/3143/

If the above does  not help, may I know the firmware and hardware version of your RE. What is the model of your home router and the network topology, and also the model of your iPhone as well, thank you

(How to find the hardware and firmware version of your devices on the Tether app ): https://www.tp-link.com/support/faq/315/#2