Why does it say weak security even though I’m coneccted?

This is most likely because your base station is using an old authentication/encryption method known as TKIP, which has been surpassed by the newer AES standard.

You need to look at your base station settings to see what (if any) options you have for the WPA2 connection settings. Use AES if you have it available. If TKIP is your only option (or you have no option at all), check if there is a newer firmware version for your router. Otherwise your options are to stick with the weaker security (and higher risk of getting compromised), or replacing your base station with one that is up to date.

Are other people in the house using an iPhone running iOS 14.1? This is a new feature and it means that the router you are connecting to has weak security. It always had weak security even before you got the alert you just didn't know about it. You need to change the security settings on your router. If this is an old router you may not be able to.

This started with iOS 14 and indicates you have an older router which may need its settings updated. While not an Apple Support Article, this is pretty good and should help --> https://tunecomp.net/wifi-weak-security-iphone-fix-ios-14/