Have you contacted you network provider to inform them you are abroad? Have you got data roaming turned on in yhe settings on your device? Could be also that you are outside of a participating network provider for your network Can't remember if WiFi calling is an option for your S9 device. But this is an option, but you will need to contact your network provider to activate this and turn it on in your phones settings
Don't know if still the case as my phone and account is always roaming enabled but if you havent turned on Roaming before you left, you won't be able to turn it on whilst overseas