My iPhone and MacBook Air say that my wifi has weak security

It means your Wi-fi router is set to a lower security type Apple now considers weak. This is a setting directly on your Wi-fi router not your Apple devices. Check your router settings and make sure they are set to WPA2 / WPA3 if possible.

If your router does not support higher security levels, then you may consider changing it.

As it is you can continue to use it, and simply ignore the message if you choose to or until you can upgrade your router.

Other devices may not consider it weak yet, or not have he option to display such messages even if they do consider it weak.

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

this means that security on your router is set to older protocols, like WPA, that nowadays considered as weak.

if possible, you should configure router to disable WPA and use WPA2, or, even better, WPA3.

in several cases you'll have to upgrade router to latest firmware to get support of newer WPA

If you call your provider, it is possible that they can fix it by changing a setting at their end