It says weak security. How do I fix that?

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

Your Wi-Fi router is probably running WEP, WPA, or WPA2 TKIP security, and Apple considers these less than secure.

How you reconfigure your Wi-Fi router to WPA2 AES, WPA2/WPA3 transitional, or to WPA3 security depends on the particular router, and also whether the router can support these connections.

Check your particular router documentation for details on how to reconfigure your Wi-Fi security.

Or ignore this, as your router is working. Just not with much security. But I’d fix this. Most routers in use can do at least WPA2 AES security. WPA2/WPA3 transitional and WPA3 router a somewhat newer router.