Monster Legends account lost

My son has been playing Monster Legends for a few years, and we've spent a fortune on buying gems and other assets/offers.

I mistakenly removed the access from my Facebook account without realising it would affect the app/account, and we've lost access to the account on the app as all we can seem to do is create a new one.

We have no record of his previous username/User ID and can't see a way to link that to/from our Apple account, via invoicing or purchases etc.

We have messaged Monster Legends, but not heard back as yet. Any ideas of how to fix/recover his User ID because as it stands, he has lost a few years of purchases/time spent?