A 3rd party app that you may have installed awhile back, probably had an update that now includes ads.
Open Google Play Store and check to see what apps were recently updated. Ones to look out for are:
Anti Virus apps
Photo manipulation apps
File managing apps
Usually there are two ways to stop ads.
Use a Firewall app - Firewall apps are powerful and if not used correctly you can block phone’s ability to make calls. Only block 3 party apps not system app, Google system apps including Google Play Store or Republic Wireless apps.
Block Background Data usage for apps, again do not restrict System apps, Google system apps or Republic Wireless apps:
Instructions to get into Safe Mode:
Press and release power button
Give Power Off a 2 or 3 second press and Safe mode option will appear.
If it doesn’t then I suggest Clearing Android System Cache. The instructions have been updated, use the ones that are easier for you to understand. The procedure is the same in both, just one might be easer to understand:
It has been a while since I took a look at this app but I keep it in the back of my mind as a possible option. While we are looking to get rid of apps that have ads, we can install one more to try to detect the ads. This app should be able to see the other apps and point out to you which ones are connected to different known ad networks to help narrow down a search to eliminate ads.