Due to Samsung investing their effort into a wide variety of devices across many different regions and under many different carriers, the company came up with a way to enable/disable certain features based on what market the device is being sold in. This is called the Country Specific Code (CSC). If you would like to enable certain features (such as VoLTE or WiFi calling) unique to your particular region or carrier, then it’s important you have the right CSC on your device. To that end, XDA Recognized Developer Tkkg1994 created an AROMA-installer that can be flashed through TWRP to allow you to select whatever CSC you want.
Note: this forum link is distinct from the earlier one for the Samsung Galaxy S8, as the CSC file that is flashed is device-specific.