Weak security

This generally means the WiFi encryption being used by the home router is something less than WPA2

If you own your home router, then you need to check its setup. If you get your WiFi router from your Internet service provider and they set the password, then you need to ask them if WiFi encryption is WPA2, and if not, then you need them to configure it for WPA2 WiFi encryption.

If your Internet service provider allows you to do that setup, then connect to your router and make those setup changes.

One other thing. If your Internet service provider is supplying your WiFi router, then if it has been a lot of years since they gave you the unit, you should ask them if they can replace it with a newer unit. That is to say if it is has been more than 5-10 years, then technology has improved a lot and a newer unit would provide better features and performance. Especially if you pay them a monthly rental fee for the router. Get them to upgrade your WiFi router. Again, this is only if the ISP provides the WiFi router and it has been a lot of years since they gave you the current WiFi router.

Since upgrading to iOS 14, many users are reporting seeing a warning on their iPhone that says their Wi-Fi has “weak security.” 

Firstly, there is no need to panic. Yes, Apple is telling you that the Wi-Fi network you’re using to connect your iPhone isn’t secure. But specifically it means that your connection is not as robust as the latest up to date standard—it doesn’t mean a hacker is inside your router and has compromised your iPhone and other devices.

Wi-Fi is governed by security protocols, which are updated to fix weaknesses in the previous iteration. The oldest (from the 1990s) and least secure is WEP. The next step up is WPA, then WPA2—either TKIP or AES. WPA2 AES is a lot stronger than WPA2 TKIP but both are a common standard used on today’s routers.

https://www.forbes.com/sites/kateoflahertyuk/2020/11/29/iphone-weak-security-wi-fi-warning-heres-what-it-means/?sh=64ec396d786a

it would help if you could be more specific as to what is happening. Two kinds of security are available. The first is a firewall, which is often provided with the internet provider's modem. Your Mac also has Firewall in it. The second is the use of passwords. I assume you have that capability. If you have a complex password to be able to access your internet, you have security. It can not be accessed without that password being entered. So, what precisely is the problem?