DAX Measure

You can try

Month on Month change =
IF (
    ISINSCOPE ( 'Date'[Year month] ),
    VAR CurrentValue = [Num trips]
    VAR PrevValue =
        CALCULATE ( [Num trips], DATEADD ( 'Date'[Date], -1, MONTH ) )
    VAR Result = CurrentValue - PrevValue
    RETURN
        Result,
    VAR MinDate =
        MINX (
            ALLSELECTED ( 'Date'[Year month sort column] ),
            'Date'[Year month sort column]
        )
    VAR MaxDate =
        MAXX (
            ALLSELECTED ( 'Date'[Year month sort column] ),
            'Date'[Year month sort column]
        )
    VAR CurrentValue =
        CALCULATE ( [Num trips], 'Date'[Year month sort column] = MaxDate )
    VAR PrevValue =
        CALCULATE ( [Num trips], 'Date'[Year month sort column] = MinDate )
    VAR Result = CurrentValue - PrevValue
    RETURN
        Result
)

Its important to use the column which year month is sorted by, rather than the year month column itself, as MIN / MAX won't work properly with alphanumeric columns. If your year month column is actually date which is just formatted in year month format then you can use that column.