Cubasis 3 Android Smartphone and MIDI USB Keyboard compatibility with/without AUDIO/MIDI Interface

although Cubasis 3.6.6. works fine on my Xiaomi Redmi Note 9 Pro Smartphone I have not been successful to make Cubasis 3.6.6. respond to MIDI from my external via 5 PIN DIN MIDI IN/OUT connected keyboards (MOTIF XS6 and KORG PA 1X PRO) neither via USB C (Smartphone) to USB A (Behringer UMC 404HD USB-Audio/MIDI Interface) connection nor via USB C (Smartphone) to USB A / USB B direct cable connection (without my MIDI Interface).
Can anybody tell me if I am doing anything wrong regarding connections and necessary adjustments/settings in Cubasis 3 here or if my old keyboards and/or my Behringer UMC 404HD are just not compatible ?
As I only need MIDI (external controller keyboard) at the moment:
Is a AUDIO/MIDI Interface necessary or is a USB C cable connection between my Android Smartphone (Cubasis 3) and an external compatible MIDI Controller Keyboard sufficient for playing a MIDI controller keyboard ?
And what about differences in latency regarding these connection options ?

I also need a smaller full-size keys MIDI controller keyboard.
Is the NEKTAR Impact GXP49 and the NEKTAR GX 49 Controller keyboard or any other similar 49 full size keys controller keyboard compatible with my Cubasis 3 Android Smartphone and is USB power sufficient or is regular power supply needed.

Thank you,

Cubasis 3.6.6. with Xiaomi Redmi Note 9 Pro Android Smartphone

Behringer UMC 404HD USB-Audio/MIDI Interface