Avec `\fancypagestyle{plain}[fancy]{}` :
\documentclass[12pt,a4paper]{report}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage{fancyhdr}
% En-Têtes
\rhead{\textsc{Haut Droite}}
\lhead{\textsc{Haut Gauche}}
% Pieds de Pages
\rfoot{\textsc{Bas Droite}}
\cfoot{\textsc{Bas Centre}}
\lfoot{\textsc{Bas Gauche}}
\pagestyle{fancy}
\renewcommand{\headrulewidth}{0.7pt}
\renewcommand{\footrulewidth}{0.7pt}
\renewcommand{\thesection}{\Roman{section}}
\fancypagestyle{plain}[fancy]{}
\begin{document}
\chapter*{title}
\clearpage
\section{Ma section}
Blabla
\end{document}\end{document}
![alt text][1]
[1]: https://texnique.fr/osqa/upfiles/Screenshot_2023-01-19_at_11.49.02.png
