Cubot Rainbow – CLEANMASTER pre-installed – uninstall ???

Hi Guys!
I bought a cheap Zweithandy in January 2017th A Cubot Rainbow.
After approximately 3 weeks appeared on the phone repeatedly saying POP up advertising and Google Chrome without saying sides opened (something with ... crapeto .... or something like that, even porn sites were present).
Unfortunately, I know myself not very good and I googled of course they were worth. I downloaded some virus scanners, etc. on Phone. Nothing helped. I did not find out which app was the culprit, although I had not installed many apps.
So I put the phone back to factory settings. I uninstalled / disabled everything I needed. U.A. Also Clean Master, who was pre-installed on the phone / is.

Now, exactly 4 weeks later, the problem appeared again. Unwanted web pages and pop ups on your home screen and in div. Other apps.
I have already searched everything. Apps permission for superimposing withdrawn etc. Nothing helped.
Then I loaded my app Trust go down. Also, since I have found nothing until I have listed the apps and have found it Clean Master.
If the CM uninstall he's back a few minutes later or after rebooting the phones.
Meanwhile I reset the phone to factory settings again. Still the same. (Angry)
So I think that the problem comes from the CM. What do you think? As of the date from which emerges the advertising, even the battery is consumed quickly .... Must I be rooting the device? 'm afraid then that I destroy any system settings.

I have read here in the forum already much, but it helped me anything yet. Now I signed up and hope someone can help me!
I say the same times a big THANK YOU to you professionals alone is for reading my novel (cool)

After all that I've noticed here in the forum already so over Clean Master and Cheetah mobile Hello, I can very well imagine that is responsible.
And if CleanMaster really preinstalled on the device is all I can say, this is really bold.

Uaaah .... preinstalled Cheetah software? So to speak malware directly in ROM !? Creepy... : D

I say it's up to the firmware. Cubot is known. And our Cubot home shows the same symptoms (Apart from the fact that Clean Master is not there)
You could Googling whether a clean ROM has been replenished, otherwise only helps ne custom ROM / Root and uninstalling the app responsible. If you can find the responsible App / a custom ROM, is another question

Thanks for the answers!
What do I need for there Googling? Know me because unfortunately etc zero from ROM with so ... "custom ROM / Root" says nothing at all to me. But I can imagine that there should be familiar reasonably well. (Thinking)

So preinstalled means for me that CM turn right the first time it was on the phone.

There as a good guide somewhere for the custom ROM?

lg Infi