Why was does our Wi-Fi say it has weak security?

iOS is alerting you to the weak security settings of your Router/WiFi access point.

You should “tighten up” your router/WAP’s wireless security settings using it’s management tools.

Apple’s security recommendations are in this article:

https://support.apple.com/en-us/HT202068

You need to update the security settings. Start here: https://support.apple.com/en-us/HT202068