Problem phenomenon:

If you have since updated your iPhone you may see the range extender’s Wi-Fi shown as having weak security. This is due to the range extender supporting the mixed WPA/WPA2 (AES/TKIP) by default for compatibility purposes. The iPhone now recognizes WPA as a weak security type. This may also be the same with other security suites with a similar notification.

Why range extender network is displayed as “Weak Security”?

A range extender is essentially a device that connects to an existing Wi-Fi hotspot and repeats the signal, so generally, it will generate the wireless security of the host Wi-Fi hotspot. While the default wireless security of the TP-Link range extender itself was auto, it supports WPA2+AES, as well as older WPA+TKIP for compatibility with older devices. Even though your iOS device recognizes the range extender network as weak security, it connects to the network with the stronger WPA2+AES by default, so there won’t be any security concerns upon to this point. 

Troubleshooting : What to do if  IOS devices  shows ' weak security'? 

Steps 1

Make sure main router is on WPA2-PSK (AES)  and IOS devices does not show' weak security' when connected to main router

Here is a related guide to check security type on router: 

Why my iOS14 warning weak security when I connect to TP-Link Wi-Fi

Step 2

Confirm Range extender is on the latest firmware : 

Download the latest firmware for range extender from tp-link Download Center or make sure firmware is up-to-date on TP-Link Tether APP. 

(select hardware version first before download the corresponding latest firmware)
 

Step 3

Restore the range extender to the factory default settings and re-associate the host Wi-Fi through the web page Quick Setup  Guidance is as below:

https://www.tp-link.com/support/faq/2485/

RESET: with RE powered on, use a pin to push the reset hole until all the LEDs flash once 

Note: if  you use Tether app to install Range extender, update Tether app in Google Play or APP store first before set up. 

If the above does not fix the problem, please report back with some information to help locate the issue:  

1.  Model of router & a photo of the wireless security settings on router

2. firmware and hardware version of range extender, better to attach a photo showing weak security

3. do you use Web or Tether to set up the extender

Note:

If some of your devices cannot connect to the whole Wi-Fi network after these modifications, you may need to change the wireless security back.

Let the community know if you have any further inquiries, thanks a lot~