"apk time recommend" pop-ups

Hi.

My android v. 5.1 keeps getting pop-up ads entitled "apk time recommend". [see pic of an example, below]. I cannot get rid of them. I cannot even find mention of "apk time recommend" on google.

I have looked in my Apps list and have been pretty liberal at uninstalling downloaded apps. I have factory reset my phone. This seemed to stop the problem for a few days. I've installed and run several virus and ad blocker apps, all of which report my phone is fine, but have no effect on the pop-ups.

Does anyone know what this is, and more importantly, how to get rid of it? Thank you.