"fj" Ligature in Computer Modern

This really needs to be addressed in the font, you could try to edit latin modern in fontforge or similar to add the ligature, but simplest is to pick a font that has it.

This is using Stix 2 text font with lualatex (or xelatex would also work)

\documentclass{article}

\usepackage{fontspec}
\setmainfont{STIX2Text-Regular.otf}

\begin{document}


\mbox{f}i\mbox{f}j

fifj

\end{document}