Cant get packages to work with yarn + webpacker + Ruby on Rails 6

Question

I am trying to make Metismenu and Bootstrap-select work on Rails 6.

I installed them both through yarn. They both show up in package.json

Placed in application.js

require('bootstrap');
require('bootstrap-select');
require('metismenu');

Enable them with

$('#metismenu').metisMenu();
$('[data-toggle="multiselect-dropdown"]').selectpicker();

However both give a error

Uncaught TypeError: $(...).metisMenu is not a function

Uncaught TypeError: $(...).selectpicker is not a function

After some digging I can't seem to find the cause of this. Anyone can help me?