The way one should handle a many-to-many relationship is to create a main form/subform design.
The main form is bound to one of the one side tables; in your case that should probably be the customer table, CustomersT.
The subform is bound to the junction table; in your case that should be Customer_joinT.
Use the master/child linking fields of the subform control to filter the records in the subform to show only the records in the junction table where the CustomerID is the same as the CustomerID currently in the main form.
You can use a combo box on the subform to provide the rows for the other FK in the junction table; in your case That would be the CustERPID so that the rows are provided by the CustomersERP_T table.
Here is a link to a properly designed sample accdb illustrating this design.