Publishing on Facebook Instant Games

Hello, we are developing a game for Facebook Instant Games Platform (messenger);

The game works fine for developers and testers and is virtually ready for submission; however, when we sent the game for review, the reviewer returned an image with error “Sorry, this feature is not supported in your region yet.” and no further explanation and/or an actual feature, that’s not supported.

The only way, we could replicate the same error was to make the FB app temporarily public and run it from a non-tester, non-developer account, that is not connected to the project in any way. And… F12 (debug tools) gives no actual errors, we don’t see any loading screens, just that message in the middle of a blank black page and no messages from whatever we print for sanity check from the first couple of lines of our index.html file…

We would apprecaiate, if someone with similar experience could help us with some guidance…

Thanks in advance.

The only external tools our project is using are limited to Firebase and FBInstant API, if that matters;

Looks like a FB issue?"Sorry%2C+this+feature+is+not+supported+in+your+region+yet."+fb+instant+games&rlz=1C5CHFA_enGB760GB760&oq="Sorry%2C+this+feature+is+not+supported+in+your+region+yet."+fb+instant+games&aqs=chrome..69i57.6458j0j1&sourceid=chrome&ie=UTF-8

Thanks; I googled for that for some time as well with little to no success…
I’ll try to contact Facebook as well, if I find the support address.