My WiFi has weak security

That message means that your router is not set to the latest security protocol. You can check your router's manual to learn how to change the settings or you can search for yout router's make and model on the internet to find that info. If your router is provided by your internet provider, contact them, you may need the modem/router updated. These are the recommended settings....

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

The weak security warning will be displayed if your router security settings are not set to WPA2 AES or to WPA3.

How you change router security settings depends on the 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 was supplied by your internet service provider, check their website for instructions.

Perhaps this article will help:

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