Samsung Galaxy Note 5 won't remember Wi-Fi passwords

[Android Central Question]

I recently got a used Samsung Galaxy Note 5. The phone was bought in the US, in a AT&T shop. The guy who bought the phone is my cousin and after using it for about 2 years he gave me the phone. As I live in Europe I sent the phone I just got to be decoded so I can use it with my mobile provider. My problem is: When I connect to a Wi-Fi network like my home, or my work network everything is fine, but when I turn my Wi-Fi off and on again I have to retype the password again in order to connect to one of these networks. The phone just doesn't remember the passwords. I tried a factory reset but it didn't help. I asked my friend about this problem but he says he didn't even use Wi-Fi that much and he can't remember if he had this problem too. The phone comes with a lot of built-in AT&T apps and one of them is AT&T Smart
Wi-Fi. I thought maybe that app has something to do with it and I tried a lot of things
with the app but nothing seems to help as I have no AT&T network here in Europe.
I thought maybe there are options in the setting of Wi-Fi of the phone. The only thing that got my eye was the option called: "Auto connect to AT&T WiFi". I tried setting this on and off but it doesn't solve my problem. This really frustrates me and I have no idea what I could try next. I hope you guys can help me.