get all models for app with ContentType

How do I get all the models for an app with ContentType?

I have this code the works on the local server but throws
"DoesNotExist: ContentType matching query does not exist" in
production:
ct = ContentType.objects.get(app_label=app_label, name=modelName)

This works:
ct = ContentType.objects.get(app_label)

So it has to be something to do with the modelName. How do can I
print out all the models for this app with ContentType.

I've tried the following at the model appears to be there:
app = get_app(app_label)
for model in get_models(app):

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