I bought a new phone in indonesia. Now I cannot make online payments to book hotels.
Hi @DavidC & welcome 
Where is the phone now (UK)?
Is it an iPhone and have you restored a previous backup, which includes the Monzo app?
Android S25. I’m in Ambon but on the way to remote islands - don’t worry. I’ll sort it out Later back in the UK
Have you set your phones region to the UK in your settings and App Store?
There could be a number of possible reasons. You’ll need to provide more detail about what you’ve done so far, the steps you’ve taken, and the specific issue or error you’re encountering.