Upvote:1
According to the US Department of State, US consular offices can authenticate foreign documents:
An authentication is the placing of the consular seal over the seal of a foreign authority whose seal or signature is on file with the United States Embassy or Consulate in order to verify the authenticity of the foreign seal or signature. A consular authentication in no way attests to the authenticity of the contents of a document but merely to the seal and signature of the issuing authority.
You'll need to contact the US Consulate in Mongolia re the process, or the USDS directly.
It appears that Taiwan is not a party to the Hague Apostille convention, which is why they're asking for authentication instead.