The problem in default Odoo is when you have many warehouse / routes. The many2many checkbox widget will only show you a certain number of routes. Desprite there are more routes.
For example you have 50 warehouses, they all supply to eachother. So that means you will have 50 x 50 = 2.500 routes. The widget can not display this plus the list will be very long.

How does this module work?

Step 1) Turn on the debug mode.
Step 2) Turn on the access right "Manage Push and Pull inventory flows"

Step 3) Go to Inventory / Configuration / Warehouses
and open one. You will see the "Resupply Warehouses" is now a list view

Step 4) Go to Sale or Purchase / Configuration / Product Categories
Open a category, and you will see that routes is now also a list view.