Why does third-party advertising appear on the site?

Question

Yandex.Direct

Good day. There was such a problem: extraneous advertising appears on the site, always different (when the page is updated). On the Internet, it is generally advised to connect an SSL certificate. When connecting SSL, hosting (reg.ru) offered to use it for a year for free (as a gift to the domain), which I did. The ads disappeared, the site had a security icon in the address bar, everything was fine. 2 weeks have passed and now the advertisement appears again, as well as messages from Yandex.Direct that problems were found with the availability of the site. What could be wrong?

And one more thing, as far as we know now, ads appear only on one device, which also displays that the site is unsafe. On other phones and PC everything is fine. Could it be because of the browser or maybe the cellular carrier?

Answer 1

If the problem is only on 1 device, then the problem is with the device)

Answer 2

it was the same on my site on a megaphone until it was transferred to https