Weak security?

Apple devices will display a weak security warning 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/mode. If you don't have instructions for your router, do an internet search for you router brand/model instructions or if your router was supplied by your internet provider, check their website. Also see: Recommended settings for Wi-Fi routers and access points - Apple Support

Simple question. Complicated potential answers. Without knowing the details about your WiFi router, an educated guess would be that it is not providing up to date security levels.

If you could provide the make and model number of your WiFi router, we'll see if we can learn more about it using Internet search tools. We do not need a serial number, so please do not provide that info. Just the manufacturer name and model number.

Apple's very good guide for WiFi is linked below to get you started.

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

Sounds like your WiFi might need a little “hardening” security-wise.

See Apple’s recommendations on the subject here:

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