There is no dedicated HYP key to call up cosh() or sinh() function in Casio fx-991EX.   This is a stern contrast to Casio fx-115ES keyboard.  It does not mean you cant use the hyperbolic funtction on fx-991EX

Hit “OPTNOption” key then select “Hyperbolic Func”

You need to be in “Calculate” mode to see this option, so for repeatability,

Hit menu, select 1.   Hit OPTNOption, select 1.

This calculator is obviously not RPN.  The calculator uses “natural notation” format.  It basically let you create equation and then evaluate the answer.  No concern on intermediate calculation.  Just answers, and it is really a wonder that you can get those devices for less than $20.