Thank you so much! it solved the problem after 2 entire days struggling. Here is minor tweak:
curl 'https://graph.facebook.com/v22.0/YOUR_PHONE_NUMBER_ID/register' \
-H 'Content-Type: application/json' \
-H 'Authorization: Bearer YOUR_ACCESS_TOKEN' \
-d '{
"messaging_product": "whatsapp",
"pin": "000000"
}'