Django authenticate always return None

Question

Found a lot of questions about it however couldn't find smth helps me. Settings.py contains Authentication Backend:

AUTHENTICATION_BACKENDS = (
    'django.contrib.auth.backends.ModelBackend',
)

And auth user model:

AUTH_USER_MODEL = 'users.User'

User model inherits from AbstractUser class:

class User(AbstractUser):
   <some_fields>

And then 3 type of users inherit from User. I registered Client:

class Client(User, models.Model):
    status = models.CharField(max_length=1, default='C', editable=False)
    <some_fields>

When I activate user by activation link everything is OK. Client is authenticated user. However when I just try to enter by login and password authenticate always return None:

class LoginView(APIView):
    def post(self, request):
        qs = User.objects.filter(Q(email=request.data.get('user')) | Q(phone=request.data.get('user')))
        if qs.exists():
            username = qs.first().username
            password = request.data.get('password')
            user = authenticate(username=username, password=password)
            if user is not None:
                login(request, user)
                return Response(CMD(user.id), status=status.HTTP_200_OK)
        return Response({
            'error': 'Invalid login / password',
        }, status=status.HTTP_400_BAD_REQUEST)

I've logged qs which returned desired Client, also username and password are ok, but authenticate returns None.

I think maybe problem is that Client isn't registered properly. This is the SignUp form:

class ClientSignupForm(forms.ModelForm):
    class Meta:
        model = Client
        exclude = [
            'username',
            'date_joined',
        ]

    def __init__(self, *args, **kwargs):
        self.request = kwargs.pop('request', None)
        super(ClientSignupForm, self).__init__(*args, **kwargs)

    def clean(self):
        <actions to validate captcha>

    def save(self):
        username = 'user' + str(User.objects.all().aggregate(Max('id'))['id__max'] + 1)
        return Client.objects.create_user(**self.cleaned_data, username=username)