I'm bit skeptical if the reminders work in Thailand. I'm on DTAC network of Thailand.
Reminders are available worldwide. Push notification reminders are generated by the app locally or, on other platforms, sent via the internet. SMS notifications also should be available worldwide 99% of the time.
You're welcome to sign-up for Premium, try it out for 30 days and if it won't work and we won't be able to make it work, you can ask us for a full refund. You can also request a refund for no particular reason within 30 days, simply if the app wouldn't meet your expectations.