\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*{Πίνακας}
\begin{table}[h]
	\begin{tabular}{|c|c|c|c|c|}
	\hline
	\multirow{2}{*}{Κατάσταση} & \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}