I am not sure i understand the problem, you have a midi keyboard in that you want to route out to the phone via android box?
As long the Android detect the two midi devices "the IOS OTG cable as midi device?" is it not just a matter of routing through IN PORT ->OUT PORT?
Any midi program probably able to do that?
But question is do the Android detect your connected devices, as midi ports?
"I have no clue howto check if midi devices connected to Android, have an OTG cable but no OTG phone, see if i can borrow my wifes and i will try".
I can only guess that there must be a setting that tells Android that the OTG connection is in MIDI mode, or is it detected?
If you have opera installed or chrome on the android, test if the webmidi API can find the devices.
https://midisequenser.000webhostapp.com