On my sister's Huawei 360 adverts appear on top of other applications whenever wifi or mobile data connectivity is on. These adverts are so annoying since the sometimes freeze the phone covering the whole screen with a small x on the right hand side corner. They also consume a lot of data. They can show even on whatsapp. I have checked if there are any launchers but only the default one appers, Also I dont any funny apps in data usage under apps that use background data. Could these apps be the one installed via USSD code or phone serial number?
The first reason for these annoying ads might be because of your browser when you have accidentally clicked on any of adds shown.Those malicious adds may install some apps in your ROM(internal memory) which may not be uninstalled so easily or will re-appear every time you connect to internet.
So, you could take backup and restore the phone to factory settings. I did the same way when i was attacked by these malicious apps.
PS: these type of apps may import your photographs and other data. Please be cautious and do factory setting ASAP.