For now, I‘m using the MidiFlow translation app on my iPhone for playing and recording Modwheel and Pitchbender with the MIDI tracks on the Digitone, repurposing the MIDI LFO and VAL8 CC settings. Also the BC setting on SYN1 page is used to switch on/off the translation (0=off), so that the internal tracks can work as usual, as needed.
For recording realtime PB slides, I found it‘s good to run the track with 2x speed, for increased resolution. But 2 bars max.
There is the Apple CCK adapter and a MIDI hub plugged into the iPhone, connecting my keyboard and the Digitone via USB.
Problem solved for me.
I uploaded the translation setup to the files section: