Hello @Chip_Oidos 

Thank you for your feedback 

May I know the model of router, the firmware version of router, and can I have a photo of the Wi-Fi settings on the main router? 

• Remove the router Wi-Fi profile on iPhone and reconnect to Router WiFi, could iPhone connect to Router SSID ?

How to remove existing wireless network profile on Android&iPhone

Then connect to Extender's Wi-Fi using the same router Wi-Fi password (RE uses the same wifi password as main router after being setup successfully)

• To isolate the issue, use an SSID for RE which is different from router's SSID

 How to change wireless settings of TP-Link range extender on Tether APP

• If router uses WPA2/WPA3 mixed, try to change to WPA2-PSK AES and see if that helps
• Try to reset iPhone network settings with Apple support 

If still the same, may I know IOS version of your iphone12 Pro? iPhone failed connect to 2.4Ghz or 5Ghz band of extended wifi? Could iPhone connect to the main router's SSID when you manually put in the WiFi password? Thank you 

Hello @Chip_Oidos 

May I know could IOS devices get internet from range extender now. 

If not, you could try to turn on the DHCP server on range extender, and set a smaller IP range on Range extender and see if that helps. 

What's the solution if my TP-Link range extender can't connect to my router properly?

To assist, better, I would also  create a support email to you, please check your inbox later. 

Hello @Chip_Oidos 

It seems that you got things work according to the support email, may I know how did you resolve it? Much appreciated if you could share detailed steps here, thank you