documentclass[12pt]{article}

usepackage{amssymb,amsmath,amsthm}

renewcommand{theenumi}{roman{enumi}}
renewcommand{labelenumi}{(theenumi)}
renewcommand{theenumii}{alph{enumii}}
renewcommand{labelenumii}{(theenumii)}

begin{document}
begin{enumerate}
    item First itemlabel{1}
    item Second itemlabel{2}
    begin{enumerate}
        item First sub-itemlabel{2.1}
        item Second sub-itemlabel{2.2}
    end{enumerate}
end{enumerate}
Reference to First sub-item~eqref{2.1}.
end{document}