I cannot access reverse ManyToMany field from django ModelForm

I have two entities with a ManyToMany relation between them. ProcessModel has a ManyToManyField of RouteModel and a reverse relalion 'processes' is created. I have define two ModelForms, but I cannot access the reverse ManyToMany relation 'processes' from the routes form. The view holding the route form don't display the processes field. How can I show that reverse ManyToMany field and save the information using the form. Here is my code

class ProcessModel(models.Model):

    name = models.CharField(verbose_name="nombre", max_length=50, blank=False, null=False)
    code = models.CharField(verbose_name="Código", max_length=50, blank=True, null=True, unique=True)    
    routes = models.ManyToManyField("RouteModel", verbose_name="lista de rutas", related_name="processes", blank=True)

class RouteModel(models.Model):
    
    name = models.CharField(verbose_name="Nombre", max_length=50, blank=False, null=False)

    route_type_id = models.ForeignKey("RouteTypeModel", verbose_name="Tipo",
                                                 blank=True, null=True, on_delete=models.SET_NULL,
                                                 related_name="routes")

    start_date = models.DateField(verbose_name="Fecha inicio")
    end_date = models.DateField(verbose_name="Fecha inicio")


from django_select2.forms import ModelSelect2Widget, ModelSelect2MultipleWidget
class ProcessForm(forms.ModelForm):
    class Meta:
        model = ProcessModel
        exclude = ('id',)

        widgets = {
            'name':forms.TextInput(attrs={'class': 'form-control'}),
            'code':forms.TextInput(attrs={'class': 'form-control'}),
            'routes': ModelSelect2MultipleWidget(model=RouteModel, queryset=RouteModel.objects.filter(),
                                            search_fields=['name__icontains'],
                                            attrs={'style': 'width: 100%;'}), 

        } 


class RouteForm(forms.ModelForm):
    
    class Meta:
        model = RouteModel
        exclude = ("id",)
        widgets = {
            'name':forms.TextInput(attrs={'class': 'form-control'}),
            'route_type_id': forms.Select(attrs={'class': 'form-control'}),
            'start_date' :forms.DateInput(attrs={'class': 'form-control datepicker', 'autocomplete': 'off'}),
            'end_date' :forms.DateInput(attrs={'class': 'form-control datepicker', 'autocomplete': 'off'}),
            'processes': ModelSelect2MultipleWidget(model=ProcessModel, queryset=ProcessModel.objects.filter(),
                                            search_fields=['name__icontains'],
                                           attrs={'style': 'width: 100%;'}), 
            }

I just found the solution, I had to add the processes field and overwrite the 'init' and 'save' functions. Here is the deffinition of the RouteForm

class RouteForm(forms.ModelForm):
    
    processes = forms.ModelMultipleChoiceField(
            queryset=ProcessModel.objects.all(),
            widget=ModelSelect2MultipleWidget(model=ProcessModel, queryset=ProcessModel.objects.filter(),
                                            search_fields=['name__icontains'],
                                            attrs={'style': 'width: 100%;'}),)
    class Meta:
        model = RouteModel
        exclude = ("id",)
        widgets = {
            'name':forms.TextInput(attrs={'class': 'form-control'}),
            'route_geom': LeafletWidget(),
            'route_type_id': forms.Select(attrs={'class': 'form-control'}),
            'start_date' :forms.DateInput(attrs={'class': 'form-control datepicker', 'autocomplete': 'off'}),
            'end_date' :forms.DateInput(attrs={'class': 'form-control datepicker', 'autocomplete': 'off'}), }

    def __init__(self, *args, **kwargs):
        super(RouteForm, self).__init__(*args, **kwargs)

        # Here we fetch the currently related projects into the field,     
        # so that they will display in the form.
        if self.instance.id:
            self.fields['processes'].initial = self.instance.processes.all(
            ).values_list('id', flat=True)

    def save(self, *args, **kwargs):
        instance = super(RouteForm, self).save(*args, **kwargs)

        # Here we save the modified project selection back into the database
        instance.processes.set(self.cleaned_data['processes'])

        return instance