\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[greek,english]{babel}
\usepackage{alphabeta}
%Times New Roman
\usepackage{newtxtext}
\usepackage{substitutefont}
\substitutefont{LGR}{\rmdefault}{txr}
\usepackage{multirow}

\begin{document}

\section*{ΠΙΝΑΚΑΣ}
\begin{table}[h]
    \begin{tabular}{|c|c|c|c|c|}
    \hline
    \multirow{2}{*}{ΚΑΤΑΣΤΑΣΗ} & \multicolumn{4}{c|}{PT} \\
        \cline{2-5}
     & $s_1$ & $s_2$ & $s_3$ & $s_4$ \\ \hline
        \multirow{2}{*}{MR1} & 0.1 & \multirow{2}{*}{0.7} & 0.2 & 0.3 \\
     \cline{2-2} \cline{5-5}
         & 0.1 & & & 0.2 \\ \hline

    \end{tabular}
    \caption{My caption} \label{tbl:1}
    
\end{table}
\end{document}

    
