I have a Samsung Galaxy S3 GT-I9300 since 2012. I've never rooted it. The last app I've installed was a few months ago. I'm running Android 4.3.
Yesterday, for the first time ever, I've got "Uninstall harmful app. 'Mobile tracker' can damage your device." notification, exactly the same as in the following thread: Uninstall harmful app. 'Mobile tracker' can damage your device.' At first, I discarded it but when it happens again and since it was from google, I said 'OK'. Later that day, I don't know if it is due to this or not but my phone started to show no signal sign and not recognizing my SIM card. I re-enabled this mobile tracker from Application Manager but still I get the same no signal sign.
When I put another SIM to my phone it works, when I put my SIM to another phone it also works. It only doesn't work when I put my SIM to my phone. I cannot understand how this can be possible, but it is the case unfortunately. Any help would be very much appreciated as I do not want to reset my phone.