Trainers and Cheat Engine methods are best left to who want granular control and are willing to accept the occasional risk of save corruption.
Go to your Far Cry 4 installation directory and find the savegames folder.
The "ULC Unlocker" (specifically the one released by the group SKIDROW) is a tool designed specifically to unlock all Ubisoft Launchable Content (ULC), including the AJM 9.
For advanced PC players, you can decrypt the game files manually using to strip the online requirement tag from the AJM 9 weapon profile entirely.
Legacy Ubisoft online services frequently drop connections, causing previously unlocked Uplay rewards to randomly relock mid-campaign.
These mods often move Uplay-exclusive items into the standard shop.
Download a "100% Unlock" or "Uplay Rewards Unlocked" starter save file.
It features a 3-round burst and supports a suppressor and extended magazine. Far Cry Wiki 2. Unlocking Without Uplay (Mods)