Sometimes my wifi connection disconnect automatically

It looks like you are connected to Wi-Fi from the Wi-Fi logo in the status bar.

Weak Security is NOT an iPhone problem and is unrelated to any other Wi-Fi issues; it is a problem with your router settings.

A "Weak Security" notification means that your Wi-Fi router is not configured correctly. There is a new feature in iOS 14 that checks and displays a message if your router is not using the most up-to-date security settings. Note that your router has always had weak security, you just didn’t know about it until now. 

See this Apple support article for how to configure it correctly—>Recommended settings for Wi-Fi routers and access points - Apple Support, specifically the section headed Security. And don't forget to configure your range extenders (if any) also!

Note also that weak security does not interfere with using Wi-Fi, it is just a warning. Your Wi-Fi should still work normally, and, if it doesn’t, it is not because of the weak security message.