Hello @Bayo 

Here are some tips that you could give it a try: 

1. Make sure IOS device could get Internet when connect to main router 

2. Try to set up static IP for range extender and set up a smaller IP range for RE and see if this helps

https://www.tp-link.com/support/faq/2965/

May I know the led status on  on the extender? To help exclude the issue, use SSID for range extender which differentcan you confirm other devices other than IOS device could get internet from Extender? How to change wireless settings of TP-Link range extender on Tether APP

Can I have a photo of IP address on IOS mobile device,and IP address on devices connected to extender? How to find IP address on your devices?