You can use an Apple Pencil (both 1st and 2nd generations) or—if you update to app version 3.54 or newer—a keyboard with mouse or trackpad with Shapr3D.
However, the Apple Pencil is the best stylus and modeling device for Shapr3D. You need the Apple Pencil to sketch and model naturally and intuitively in Shapr3D. This is only possible because the Apple Pencil has a pressure sensitivity and low latency that most other styluses don’t have.
You can find out which iPads are compatible with the Apple Pencil here.