Monzo doesn't have a Google Pay or Debit Card top up option. You can sign up for a Revolut account which has the option to topup with Google Pay. Add your Curve card to Google Pay and topup money that way. Revolut is very similar to Monzo but you can just EFT from your Revolut account to Monzo...
Method I've tested is by getting a Curve card in the UK, then adding your Global Account to it and changing the default currency on the card to GBP. You can then topup your Revolut Account (similar to Monzo) with Google Pay using the Curve card with your FNB Global Card selected. No extra...