Hi, @Blondie63, and thanks for the update. A Factory reset might end up being the best solution.
I do have some other suggestions before you do this, however.
It would be a good idea at this stage to pop your S21 in to Safe Mode. This will disable any third-party applications, so that we can determine whether these are causing the issue.
To do this, press and hold the Power button until you see Power off, Restart and Emergency > Press and Hold the Power Off icon until it shows a Shield > Tap the Shield. After this, the S21 will start in Safe Mode.
If the pop-up ads are not evident whilst in Safe Mode, then it suggests that these are being generated by a third-party app. To resolve this, exit Safe Mode, then begin uninstalling apps you've downloaded one by one (restarting the phone between each app uninstallation) until the issue fixes itself. Once you've identified the troublesome app, you can reinstall the other apps via the Galaxy/Google Play Store.
Should the issue persist, I recommend the next step.
Sometimes while browsing the web, an ad will take over your screen and tries to scare you into thinking your phone has been hacked, or otherwise prompts you to download other software. You can use Ad blockers in Samsung Internet to prevent pop-ups. Navigate to and open Samsung Internet, and then tap Menu (the three horizontal lines). Tap Ad blockers, and then choose the services you’d like to use. Tap the Download icon to begin downloading.
Please let us know whether this helps.
