# Packages does not load in child files with standalone

I'm trying to start my first modular Latex file and I'm using the Wikibook of Latex. I don't know if I'm doing something wrong but I tried to find info about it and I didn't find anything.

I'm using the standalone package and the packages in the child files are not loading. In this case, the package \usepackage[showframe]{geometry} is not loading. The package only loads if I include it in the main file or in the sty file. What I am doing wrong?

These are my files:

main.tex

\documentclass{book}
\usepackage{standalone}
\usepackage{mystyle}

\begin{document}
\input{./tex/dev.tex}
\end{document}


dev.tex

\documentclass{standalone}
\usepackage{../mystyle}

\usepackage[showframe]{geometry}

\begin{document}

\begin{multicols}{2}
\begin{align}
f(x) &= x^2+uv+u^3xy
\end{align}
\end{multicols}
\end{document}


mystyle.sty

\ProvidesPackage{mystyle}

\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{mathrsfs}
\usepackage{amsfonts}
\usepackage{tikz}
\usepackage{multicol}

\newcommand{\Tau}{\mathcal{T}}
\renewcommand{\r}{\mathbb{R}}
\newcommand{\q}{\mathbb{Q}}


As per the standalone documentation:
Thus, it is a requirement to have the main file load all the packages that are required by the sub-files. My recommendation would be to put the \usepackage[showframe]{geometry} in the mystyle.sty.