Does CoreESIM function in non-EU countries bordering Lithuania, like Belarus or Kaliningrad (Russia)?

No, this CoreESIM plan is strictly geo-fenced for use within the European Union. Consequently, it will not provide data connectivity in non-EU territories adjacent to Lithuania, such as Belarus or the Kaliningrad Oblast of Russia. If your travel plans include these regions, your CoreESIM service will cease at the border. Attempting to use a standard EU CoreESIM in these areas would result in a complete loss of service or potentially trigger exorbitant out-of-plan roaming charges from underlying carriers. A separate regional or country-specific eSIM would be required.