Perfekt.

That's the code I used:

\documentclass{report}

\usepackage[a4paper,left=2.1cm, right=2.1cm,top=2.9cm, bottom=2.9cm]{geometry}

\usepackage{fancyhdr}
\usepackage{xcolor}
\usepackage{lipsum}
\usepackage{titlesec}

\titleformat{\chapter}[display]
{\normalfont\bfseries}{}{0pt}{\Large}

%General settings
\pagestyle{fancy}
%\fancyhf{} remove default page numbers
%\fancyfoot[R]{\thepage} page numbers on the bottom right side of the document
\renewcommand\thesection{\arabic{section}}
\renewcommand{\thechapter}{\Roman{chapter}}

\begin{document}

\begin{titlepage}
\textcolor{red}{\Huge
Titlepage
}

\thispagestyle{empty}
\end{titlepage}

\clearpage %makes sure that the content starts on page 2

\tableofcontents

\chapter{Part one}
\lipsum[2-4]

\chapter{Part two}
\section{Section 1}
\lipsum[4-16]
\section{Section 2}
\lipsum[4-8]
\section{Section 3}
\lipsum[4-8]
\section{Section 4}
\lipsum[4-8]
\chapter{Part three}
\lipsum[2-8]
\chapter{Part four}
\lipsum[2-8]
\end{document}


But the problem is: Chapter writes the word "Chapter N" and removes my header. I want the Chapter to behave like a section, but with roman numbering. There is a solution that removes the "Chapter N": How do I remove "Chapter N" from the chapter titles of a book

But I still have the problem that the spacing is different and my headline does not show up at the page where a chapter starts. How can I do this?

Thank you very much.

• Welcome to TeX SX! Since you use titlesec, see in the documentation § 6 Contents: The titletoc package, pp.14–20. – Bernard Nov 27 '19 at 15:15