I did a search and found the below post from one of the forum dudes put up in May 2018 -looks like you require OL access to enable it though -perhaps troop yer PC somewhere and do that and then troop it back again;
The new OFFLINE mode is now implemented (In version 2.5.1 and higher). Users can turn on the OFFLINE mode which permits the game to be used without internet identification and without a time limit. Note: users must turn off the OFFLINE mode on the same PC. OFFLINE mode cannot be turned off from another PC even with the same user login.
To transfer DCS to offline mode, click on the button in the top panel of the main menu. This can be done ONLY after successful authorization. You cannot enable Offline mode unless you have an internet connection first.
The mode is unlimited in time, thus all network services (Manager of Modules, multiplayer, news) will be inaccessible.
Exiting offline mode is possible only on the same computer* on which Offline mode was entered, it cannot be transferred to another computer.
* "the same computer" - means exactly the same both in hardware and in BIOS, Windows. For example, changing the BIOS in offline mode will no longer allow you to log in.