Welcome to the Community.
To fix your split keyboard at the bottom of the screen, start with a full on-screen keyboard.
With the full keyboard on screen, touch-and-hold the keyboard key at bottom right; a set of options will appear - slide your finger to the “dock” option, then lift your finger to fix the full keyboard at the bottom of your screen.
Next, still with the full keyboard on screen, touch the centre row of keys with two fingers - and while keeping your fingers in contact with the screen, spread outwards in opposite directions towards the left/right edges - and release.
If you follow these steps, you should now have a split keyboard - but fixed to the bottom edge of the screen.
NB: If you select Split Screen from the keyboard icon, the Split will float - and not be fixed at the bottom edge.
Hope you find this helpful in resolving your frustration.