Advertising on a smartphone of unknown etiology. What is the reason?

Question

Android

Good day. Do not kick right away and do not send to read mana - a question from a practically lamer.
There was and continues to be incomprehensible garbage. My husband bought a smartphone via the Internet, smart actually Lenovo Vibe X3. The device itself is good and serviceable, but with some kind of heresy with the firmware. If the Internet on the phone is turned off - everything is ok. Just turn it on - everything, advertising fell down. Push notifications, pop-ups, all that crap. At the same time, there are only custom applications, a viber with Skype, and a toy that has nothing to do with the matter, since advertising was pouring in even before it was installed.
Three questions:
- what could it be;
- where it sits;
- how to get rid of it.
I will not state my conjectures about the roots of this evil, since they are stupidly absent. No, I can assume that it was the unfriendly Chinese who stuffed such a "bomb" into the phone, but this still does not give me a solution. The firmware is still native, they haven’t touched it yet only for the reason that my husband has two SIM cards, and the data is all clean on the phone, then for a long time to merge everything on the computer and back. And different ed-blocks did not give the effect, as well as antiviruses that do not see anything, as far as I could understand.
Actually, help in any way you can.

Answer 1

In my almost similar situation (a freshly flashed phone sent SMS, apparently to paid numbers, but they did not go through, since the numbers were Chinese) Kaspersky helped. By the way, there was also Lenovo, Vibe Z. No third-party applications were installed, the module was in the firmware.

Answer 2

Reflash to normal firmware. Leave the Chinese to the Chinese. What and how - look at w3bsit3-dns.com. If you are not confident in your abilities, then contact the service.