I’m sorry to report that: using the ‘CMD’ key on my iPad Pro 2024 (with Magic Keyboard), the test notes in a bar on the bass staff moved to between the staves (using a grand staff), but further presses failed to move them to the treble staff. 
Trying again without the Magic Keyboard, I tried typing N into the tools popover (just above the fingerings popover) but nothing happened. now to find out what that popover is for! 
(cue the pink panther theme)
You should find that you can cross notes to the staff above simply by selecting them and hitting N with no modifiers: just the N key on its own. Likewise, to move to the staff below, use M. You can also use the contextual menu on the secondary toolbar, where you will find a Cross Staff submenu, and Cross to Staff Above and Cross to Staff Below in there. (You can also access these commands via the jump bar.)
(Edited to correct my daft mistake!)
(I believe it’s N for crossing above - think “north” - and M for crossing below - like how the middle of a capital M points downwards.)