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Router expects to get viewsets and not api views. Since this is a classical retrieve api i suggest you use the mixin for it. this is how i would rebuild you code.
@permission_classes((permissions.AllowAny,))
class ProductInfo(RetrieveModelMixin, viewsets.GenericViewSet):
queryset = models.Product.objects.all()
lookup_field = "slug"
serializer_class = CitySerializer
Source:stackexchange.com