Not sure where OP is based out of, but if US based I don't think any coverage applies outside US (vehicle and liability).

Credit card vehicle coverage may apply but you have to check terms and conditions to see if there is a clause that invalidates if underlying rental agreement is breached (also check rental agreement to see if driving into those countries constitutes a breach)

However that only deals with the Sixt vehicle; it does not address any liability for damages you may cause to others. Operating without that coverage may be illegal.