Change security to WPA3

The change is made on your Wi-Fi router. Exactly how varies by router brand/model. Many routers don't support WPA3 but WPA2 AES will also make the weak security message go away. If you don't have instructions for your router, do an internet search for your router brand/model or if your router is supplied by your internet service provider, check their website for instructions.

The Wi-Fi chip in your iPhone 11 would need to support the WPA3 security protocol as would your router where you configured the security protocol to be used in that SSID that you connect too. Your choice of password complexity and length may also cause a security warning if it is too short, or not comprised of the character mix necessary for strong encryption.