seo – Suddenly position fall in Google

I managed a website last year, and suddenly (after August) we had a problem with our position in Google.

Seo problem

In August, we went down to position 85, so I went back to running the seo website (with the Yoast add-on) and finally we went after 1 week on the second / third page.
But yesterday and today, without any change, we have come back down on the eighth page.

I want to inform you that I am not a SEO expert, so let me know what I should do next, or what tools to use / learn.

Thank you!