Using the Trellis M4 with an iOS synthesizer, such as the excellent AudioKit Synth One, is easy, but you will need an adapter to connect the USB output of the Trellis M4 to the Lightning input of the iOS device.
The iOS Lightning to USB OTG cable allows you to connect the Trellis M4 right to your iPad, iPhone, or iPod Touch. The device will also provide power to your Trellis M4!
Depending on your software, you may need to select and configure the Trellis M4 as the chosen MIDI device.
Synth One found it automatically, but you can use the MIDI Learn function to assign the x-axis (forward/backward) tilt to any knob.
Press the "MIDI Learn" button
Tap on the knob you want to control -- Here I've chosen the Filter cutoff frequency
Tilt the Trellis M4 forward and backward. The range doesn't matter, it just needs to register that that MIDI CC number (1 in this case) is the once changing
Press the "MIDI Learn" button to finish
Now, as you tilt the Trellis M4, the Filter cutoff frequency changes! Have fun exploring different parameters as you play.