I don't know if this will help, or just make me look like a tool. But just in case I'm going to do a step by step on how I get this trainer working every time. N.B uPlay version only, Steam may differ.
Here goes:
1. Open uPlay
2. Click menu -> Go offline
3. Disconnect from the internet on uPlay relaunch
4. Go into game directory (e.g X:/uplay/GhostReconWildlands) and launch game with GRW.exe
5. Wait for game to load, you will get offline complaints from the game but continue.
6. When your characters appear on screen after starting the campaign, pause, and then ALT+TAB back to desktop.
7. Open the Trainer, but DO NOT close the DOS prompt.
8. ALT+TAB back to Wildlands.
9. Enjoy
10. Upon finishing the game, go to the DOS prompt and press the enter key, then close trainer and restart internet and PC.
Maybe some of you guys do this, maybe another way, but I'm sure some of the complaints will be user error.