Turning off data roaming on the phone is not enough.
For Singtel, have to turn off data roaming (Network) to be safe.
When I travel, I want to use the eSIM that I bought from Shopee, because it is cheaper.
I onboarded the eSIM and switched that to data only while my primary line is for calls.
For my primary line ie my number, I turned off data roaming for that SIM.
Went to South Korea and still kenna some overseas data usage charges.
Called Singtel when I came back and they reversed the charges.
They told me have to turn off data roaming at their end too to be sure.
So if you have the Singtel App on your iPhone, Open the App.
Under Home -> Play details -> Settings (the gear icon) -> Data Roaming (Network)
I went to South Korea again doing that and did not encounter the same problem anymore.