phone disconnects from wifi and reconnects when i watch a video on youtube y/o check an application

 I recently bought this phone, and I have problems because when I'm watching a video, it disconnects from the wifi and reconnects, and so on, I can not see the video corridamente or fully. 

Also this not only happens to me with youtube videos, it also happens to me with applications, for example I'm browsing and it disconnects, and reconnects after 15 seconds, and the same thing happens again seconds later. I want to know how to solve this problem that worries me because with another phone, the nokia 1 plus does not happen to me.

Thank you.