To successfully update your virus signature database without a direct connection, follow these steps:

ESET NOD32 highly regarded for its offline update capabilities

A standalone utility that downloads detection engine updates from ESET servers to a local folder or network share. Update Mirror (Internal):

An offline PC still needs to verify its license. If the PC is strictly offline, you may need to perform an offline activation (using a license key provided by ESET support or your admin) rather than the standard username/password login which attempts to ping the server.