@smikula  Hello, 

RE505X should synchronize the wireless security and encryption if router is on WPA2/ WPA2 _AES.

Confirm your Tether APP is latest in APP store or Google Play if you use Tether to set up the RE,  factory reset range extender and set up again.

Confirm iPhone does not show weak security when connected on main router.

If still the same issue, may I know the model of router and can I have a photo of the wireless security settings? Can I have a photo showing weak security, do you use Web or Tether to set up the extender, thank you.

Related thread for reference:iPhone iOS14 recognizing the range extender wireless network as "weak security"

https://community.tp-link.com/en/home/forum/topic/232218