How do I fix weak security on my iPhone

Change the security settings on your router to WPA2 AES to to WPA 3. How you do the varies by router. If you don't have instructions, do an internet search for your router brand/model or if your router is supplied by your internet service provider, check their support website.

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

You don’t fix it on the phone. You update the security on your router.