run the app on the phone while connected to the computer
you will see some debug messages from unreal
the ones starting with artoolkit are related to the plugin
Also can you please check your phone’s file system ? There should be /storage/emulated/0/UE4game/YOUR_APP_NAME/YOUR_APP_NAME/Content/ARToolkit/Data and DataNFT
If those directories are missing add them in your project’s package settings -> additional non asset directories to copy