You need to use a recent Wireshark version with implementation for nr-rrc dissectors (Wireshark version 3.0.0 at least).
$ mkdir -p ${XDG_LIB_HOME:-~/.local/lib}/wireshark/plugins
$ git -C ${XDG_LIB_HOME:-~/.local/lib}/wireshark/plugins clone https://github.com/nextmn/RLS-wireshark-dissector
Alternalively, if you are using Debian Bullseye, I made a .deb package. It is available at deb-royer.irit.fr (repository installation instuctions are on the website itself) under the name wireshark-ueransim-rls. This package may not work if you have build yourself and installed Wireshark from source (because plugins folder path may not be the same).
This plugin is licensed under the Creative Commons Zero v1.0 Universal licence (CC0-1.0).
If you have any issue running this plugin, please consult the Troubleshooting page on the wiki.