Just make sure that the Offline computers are able to reach the Server/Machine which has the HTTP Proxy installed on over port 3128.
Now make sure that the Server/Machine with the HTTP proxy installed has a clear line of site to the following list of ESET servers found here: https://support.eset.com/kb332/#era6
-Download detection engine and module updates
-Download product installers, updates
-ESET Live Grid
-ESET Data Framework
-ESET Push Notification Service
Now, make the necessary HTTP Proxy policy configuration changes from the ERA/ESMC and apply to all machines you wish to have updating locally.
Agent Policy -> ESET Remote Administrator Agent -> Settings > Advanced > Use HTTP Proxy - Set to HTTP Proxy address. For the download of Security Software installation files (ESET Remote Administrator Agent/Advanced Settings/HTTP Proxy) Endpoint Antivirus Policy > ESET Security Product for Windows > Advanced Settings > Tools > Proxy Server - Can use either Corporate Web Proxy address or internal HTTP proxy address. Used for client traffic to the internet (Live Grid, Activation etc.) -
(In your case do not set as Corporate Web Proxy as this will create issues with your offline licencing.) Endpoint Antivirus Policy > ESET Security Product for Windows > Update > Profiles > Connection Options > Proxy Server - Set this to internal HTTP Proxy. Used for Virus Signiture updates
Please make sure the Server/Machine with the HTTP Proxy installed has a clear line of site, this includes no proxy chaining as it is not supported.