In mobile phone, long touch the cell with formula and touch Copy, then drag the "fill handle" to the end of the column and touch Paste Special>Formula Only
you can even change the header to '=arrayformula({"SUM",A2:A+B2:B})' and it will put the formula in the header so you can filter and order the numbers below without messing with the formulas.
I don't have a square option that appears.. I only have a dot, when I double click it, it doesnt do anything :( I do have a formula in the cell that is functional