[Android Central Question]

I have a UMX U673C phone. I keep getting ads popping up on my lock screen. I've tried everything I read. No pop up's when I'm using the phone. Just on my lock screen. I deleted all my down loaded app's. No luck. I factory reset the phone. Nothing was downloaded but I still get the annoying ad. Before I reset the phone I downloaded multiple adaware ad anti virus app's. Honestly they all were pretty lame. Eventually I'll get a new phone. In the mean time I would like to learn how to solve this. Thanks in advance for any help.