Hi @Deee,
The following security modes have been considered insecure by Apple. So when IOS devices detect any of them was used by your router, the weak security will pop out.
( https://support.apple.com/en-us/HT202068 )
WPA/WPA2 mixed modes
WPA Personal
WEP, including WEP Open, WEP Shared, WEP Transitional Security Network, or Dynamic WEP (WEP with 802.1X)
TKIP, including any security setting with TKIP in the name
The RE220 supports WPA-PSK / WPA2-PSK, so it is normal to see the warning.
(1) it is recommended to change the encryption method of the front-end router or main AP to WPA2 + AES;
(2) then upgrade the RE220's firmware to the newest in the official support page;
(3) then reset and reconfigure the RE220 after upgrading to the newest firmware. Set the RE's SSID to a new one for checking, it's more recommended to configure through the webpage. Then Reboot the router and RE220 together after that.