I was this sissue, my ZTE Blade A6 connects 5 min and disconnect other 3, and so on, and I found other solution.
The ADSL modems has two or more WLAN radios, mine one have four, and I did activate the second oner, put a name, password, and other parameters, and save it.
After that you can see the another WiFi signal in your smartphone, tablet ...
I had this issue and all online guides failed, i eventually downloaded kingroot and it fixed it when i did "optamize" in that program and "activate game mode". If you have any battery saving apps or use kingroots Purify sub-app, do not "optimize battery" as one of the things it does is affect the wifi as that is a battery drain, so switch off any "optamize ...
Solution- Keep IPv6 turned off.
Reason- ISP not providing IPv6 connection so while it's on in the router settings it somehow stops my access to certain apps only on my redmi note 3.
Yeah IDK but it's good enough for me.