I’m getting a notice that my WiFi has weak Security. How do I fix this ?
First, make sure that is not some kind of a scam.
Second, see the 'Weak security settings to avoid on your router' (and others) section in:
Recommended settings for Wi-Fi routers and access points - Apple Support
What is the operating system on your iPhone?
You may be securing your router with an older protocol, and the phone's operating system is telling you to choose a stronger security system. Think of it like this example: For years you have been locking your front door only with the lock that is on the doorknob. Recently, the police sent out a community announcement that there has been an increase in crime in your area, and they advise that you install a deadbolt on your front door. Your door still locks, but it might be a good idea to get a deadbolt installed. That "weak security" announcement is like the police announcement.
Assuming that this issue is happening in your home, contact your internet provider. They should be able to help you set up a newer security protocol. Here's a thread on the subject from the Apple Community.