So apparently the suggestion in here doesn't work in my document. I want that all enumerate and nested ones always uses arabic numerals, from 1,2,etc.
My preamble is this.
\documentclass{article}
\usepackage[version=3]{mhchem} % Package for chemical equation typesetting
\usepackage{siunitx} % Provides the \SI{}{} and \si{} command for typesetting SI units
\usepackage{graphicx} % Required for the inclusion of images
\usepackage{amsmath} % Required for some math elements
\setlength\parindent{0pt} % Removes all indentation from paragraphs
\usepackage{colortbl}
\newcommand{\comrem}[1]{\textcolor{red}{#1}}
\renewcommand{\labelenumi}{\alph{enumi}.} % Make numbering in the enumerate environment by letter rather than number (e.g. section 6)
\renewcommand{\labelenumii}{\Roman{enumii}}


a.,b., &c. and the second level will useI,II, &c. – Bernard Feb 07 '20 at 20:19