My WiFi is showing Weak Security

Apple devices will display a weak security warning if the router security settings are not set to WPA 2 AES or to WPA 3. How you change the settings varies by router brand/model. 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.

Also see: Recommended settings for Wi-Fi routers and access points - Apple Support