I need to move the NPM v8.1 from old server to new server.
But both of old & new server don't connect to Internet.
How can I migrate licenses without internet?
Short story is that as long as YOU can get to the internet, you can un-register a license and re-register it to a new machine using the customer portal (and a ticket to customer service for the un-registering part).
NOW... that said I don't know if 8.1 is still supported. You also didn't say if you had an active support contract. So your mileage may vary.