Looks like the deal only works for trips in the UK, I received my reply back from Uber:
"I took a look at your trip and it appears this promo was tied to a specific geographical zone you weren’t in at that time, which explains why it was not applied automatically, sorry about that!
As a reminder, this promo code works only for trips in UK."
Just to reiterate my previous response, the code is only valid in the UK. This is clearly mentioned in the ‘Promotions’ section in the Uber app.
I am also currently abroad it looks like payment processing is routed to the nearest geographical area you’re in i.e. in the US it’s processed in San Francisco, CA. I can’t explain why you’re not able to use your mondo card for Uber in the states, I’ve used it the other day and worked fine. Would be useful if you have multiple cards linked to your uber account for back up in case you encounter any difficulty using it.