@Zorki48 

Thanks for your feedback about your issue.

I guess you are talking about the "weak security" warning on your IOS 15.3 Iphone right?

Don't worry. It does not involve any security issue.

Ever since IOS 14 updates(https://support.apple.com/en-us/HT211808 ), Apple has made great changes on the security update on IOS devices. New Privacy features improve user transparency and control over how apps access your location, photos, microphone, and camera. After that, sometimes you might get a "Privacy warning" or "Weak Security" on your Wi-Fi network.

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

So solution for your issue is to set security type as “WPA2-PSK[AES]”. After that, there should be no weak security warning appearing on the mobile phone anymore. 

For more information about weak security warning, please refer to below FAQ:

The Weak Security and Privacy Warning on the iOS14 or iOS15