@VCBaldwin 

It may be with the recent Apple update in that wireless networks running TKIP are label as not being secured in favor of AES networks.

What you can try to do is to factory reset the wireless extender, then in your router change the 2.4G so that the wireless security is AES only, not TKIP+AES. Then use your web browser to set up the TP-Link range extender again.

@Tony 

Thank you for the advice.  I will try this and let you know if I have any issues.

Cheers,

@VCBaldwin I'm not sure your Extender supports AES.    I have TL-WA855RE and do not have a specific option for AES (when choosing WPA2)

Refer to: https://community.tp-link.com/us/home/forum/topic/230806 (no solution, just info). 

@Mobial I agree with @VCBaldwin since these are creating new networks from your routers WiFi. You can name your SSID's totally separate

from your regular router and use the provided encryption tp-link provides that is from 2018. Tp-Link should provide updates to their firmware ASAP. There are YouTube videos that can show any amateur or script KIDDIE  how to use Aircrack-NG in Kali Linux. Then, while using a G1100 router (basic Fios procided Gateway) it's just a matter of doing math to crack someones provided wifi Pre Shared Key (PSK) with a rainbow table. The normal consumer doesn't know a lot, if anything about wifi security. They just follow your setup steps. Where can I submit a Proof of concept for Bug bounties? Security issues seem to be in many other devices besides your wifi extenders. 

@Tony I am getting the same message. This should go to to-link. Apple says I should look at to-link for the reason for weak security message 

Thanks  everyone for your very useful advice.  I've invested in longer cables and relocated my router to a more central area in my apartment which has improved the signal and turned off the TP Link extender.  It's at least 5 years old so I'll purchase a newer model if the signal weakens again.

@VCBaldwin