I had this issue since a month where a full screen Ads from YMGT.xyz shows every time and then. I tried many tools and i deleted all the apps that i recently installed but with no luck..!
Yesterday and during google search for an ultimate fix, i found the below link and it was really the one I am looking for.
After applying all the steps, i was surprised that the app causes the problem is: ai.type keyboard which i had used for years and never suspect about. Now it is almost 72 hours since i deleted it and since then no more annoying Ads.
Note: my device is Galaxy Note 8
hope this helps..