A 400 Bad Request with a Django application usually points to an ALLOWED_HOSTS configuration issue, as you mentioned. However, It’s hard to help without specific details.
Please could you share some specific details of the issue you are experiencing that may help us troubleshoot it with you, e.g. the actual custom domain, URLs where you see the error, etc.
If you don’t want to share these details on the community forum, please feel free to raise a ticket from the “Help” or “Contact Support” links while logged into the Dashboard.