I got this phone two years ago and since the start this has happen. At the start it was saying "Internet may not be available", but now it says "Connected without Internet".
Yes i have reset network settings, factory reset, restarted router, changed channels in router settings, this happens every 2 minutes. Lets say I start watching youtube, on the right side of the Wifi Icon it appears a ! and when you open wifi its says connected without internet.
I'm using mobile data and while I'm using mobile data this problem doesn't happen. But I can't watch videos/play games/work on Mobile Data, i only got 25 GB, 4G internet every month.
By the way I even got my phone changed from where i bought it and the problem still happens. None of the updates has fixed it.
Edit: This happens only to my phone, all of other phones work fine. THIS HAPPENS ON EVERY WIFI NETWORK