Using trainers in multiplayer is highly discouraged and can lead to bans or instant desyncs between players.
Since the game is now "abandonware" (not officially sold on digital storefronts), the best places to find reliable trainers are long-standing community hubs: Battle For Middle Earth 2 - Rise Of The Witch King Trainer
Most modern players use the All-in-One Launcher to manage game versions. Be sure to select the correct game version in the launcher before activating your trainer. Using trainers in multiplayer is highly discouraged and
Makes your units and heroes invincible to enemy attacks. Battle For Middle Earth 2 - Rise Of The Witch King Trainer