Indigo Airlines — Unable to use IndiGo Shell Credit for new booking

After canceling one-way tickets and putting the refund in an IndiGo Shell Credit account, IndiGo customer agents are unable to apply the credit to my new booking.
I have spent many hours on the IndiGo website, emails, WhatsApp messages and telephone calls to make the new booking. However, their agents have assured me they can sort this out but they haven't done this and the telephone line mysteriously gets cut off each time I tell them the credit has not been applied on the account.
The website authentication system (SMS codes) does not work with non-Indian mobile numbers.
I now have to get a refund from my credit card company who will do a recharge to collect the money from IndiGo.
A very disappointing customer experience.