Weak security

The related article link at the top of this page, which is the very article which brought you here, offers what you should do to update your router. You may need to get your Internet Service Provider to help with this.

That message is telling you that the security setting on your WiFi router is weak, not on your iPhone. Your security has been weak the only change is an update in iOS now tells you it is weak. Check with your ISP or your router manual to set it to a more secure setting or if it is an old router get a new one.