Resume nested enumerate lists in beamer

Question

In beamer, I want to create two nested enumerate lists and continue the numbers without starting from the beginning like so.

1. 
2. 
     3.
     4.   
5.
6. 

My code is based on this solution https://tex.stackexchange.com/a/531803/153274.

\documentclass{beamer}
\newcounter{currentenumi}
\begin{document}

\begin{frame}
    \frametitle{Plan}
    \begin{enumerate}
        \setbeamertemplate{enumerate items}[square]
        \item {\large Summary}
        \vskip1em
        \item {\large Research}
        \setcounter{currentenumi}{\theenumi}
        \vskip1em
        \begin{enumerate}
        \setcounter{enumi}{\thecurrentenumi}
            \item {\large Introduction}
            \vskip1em
            \item {\large Research During PhD}
            \vskip1em
            \item {\large Research During PostDoc}
        \end{enumerate}
        \vskip1em
        \item {\large Conclusions}
    \end{enumerate}
\end{frame}
\end{document}

I tried this solution https://tex.stackexchange.com/a/55001/153274 and it did not work too.

Answer 1

As you want to change the counter of a second level enumeration, you need enumii instead of enumi:

\documentclass{beamer}
\newcounter{currentenumi}
\begin{document}

\begin{frame}
    \frametitle{Plan}
    \begin{enumerate}
        \setbeamertemplate{enumerate items}[square]
        \item {\large Summary}
        \vskip1em
        \item {\large Research}
        \setcounter{currentenumi}{\theenumi}
        \vskip1em
        \begin{enumerate}
        \setcounter{enumii}{\thecurrentenumi}
            \item {\large Introduction}
            \vskip1em
            \item {\large Research During PhD}
            \vskip1em
            \item {\large Research During PostDoc}
            \setcounter{currentenumi}{\theenumii}
        \end{enumerate}
        \vskip1em
        \setcounter{enumi}{\thecurrentenumi}
        \item {\large Conclusions}
    \end{enumerate}
\end{frame}
\end{document}