If you use the steam version, start steam offline or turn off your internet if that makes you feel safer.
Okay, you need cheat engine for this:
1. Start your game offline, don't go into the menus yet, stay on that "press enter to start" screen
2. Open Cheat Engine now and select your game on the process table.
3. Scan for text: FRLobbyLANGame , when done, four addresses should appear.
4. Select all of them and change them at the same time to ZMLobbyLANGame if you want to play zombies or MPLobbyLANGame if you want to play multiplayer offline.
5. Go back to your game and select "Free Run" mode, instead of opening Free Run, it will open the mode you have chosen.
I hope I have helped, if I didn't, there are some videos on the internet on tutorials on how to do this, also keep in mind, changing the code of the game is against the rules, don't even think about doing this online.