Hi zooranium,
To prevent Cubasis from playing back internal instruments, please tap the instrument icon in of a track and choose “No instrument”. That way you’re able to limit MIDI playback to external app instruments.
Please note that Cubasis LE comes with a limited feature set compared to the full Cubasis version: https://www.steinberg.net/en/products/mobile_apps/cubasis_le/feature_comparison.html
If you still fail to connect external app instruments, intended limitations such as the lack of Audio Unit, Inter-App Audio and Audiobus support could be the reason.
To get over these limitations, please have a look at the available full feature set in-app purchase (located in Cubasis LE’s in-app shop). This IAP updates Cubasis LE to the same feature set of the full Cubasis version at a reduced price.
Hope that helps.
Best,
Lars