No internet error

[AC Question]

I recently rooted my phone. I've noticed that when I'm connected to a wifi network, I get a notification that there's no internet and that I should connect do a different network, even though internet (wifi) works just fine. It also shows a little exclamation mark next to the wifi reception-bars, in the notification bar at the top of the screen.
I did a speedtest and still get about the same speeds as before.

Phone: Samsung Galaxy S6 Edge (G925F)
Android version: 7.0