Hi! Is it possible that, when adding a required field, you caused >100,000 documents to become unsyncable (as described here)?
Regardless, if possible I would recommend terminating and re-enabling Sync. That will most likely get you out of this situation.
In the future. if you want to add a required field to your schema without causing Atlas documents to become unsyncable, you can use the default property in your App Services JSON schema. This will attach the default value to any Atlas documents missing the required field. See more here