Can you please clarify exactly what you mean by "I'm on Singtel/EE network here".
EE is a UK network, but roaming allows you to connect to local networks in foreign countries, which in turn allows you to make & receive calls, send & receive texts, and use mobile data. Forget your billing add-ons to cover the basics here. If you don't enable roaming on your SIM/account, you can't connect to foreign networks and the whole process fails.
If you have enabled roaming on your SIM/account - usually by texting ROAMING to 150 before leaving the UK - then you have permissions to connect to foreign networks and the rest of the process naturally follows. "Data roaming" is just a phone setting to allow use of mobile data whilst already roaming. Texts & calls don't use data, and neither can you "data roam" if you can't roam in the first place.
So - fundamental question - does your phone show a connection to a local network? How exactly did you purchase the billing add-on when you arrived? (I'm trying to establish whether you have a network connection or if you used local WiFi)
Have you tried performing a manual network search and cycling through each local network in turn to see if the symptoms are the same? If you are confident that you have enabled roaming or that you've roamed before, then that's the standard fix to work around potential network-specific issues in foreign countries.
