You would need to use the addMonths function on a date to get the next month.
addMonths([%CurrentDateTime%], 1)
https://docs.mendix.com/refguide/add-date-function-calls/#8-addmonthsutc
Once you have that, you can then use formatDateTime to get the month. If you wanted this as String and assuming you created the next month in the variable $nextMonth, you could do something like this...
formatDateTime($nextMonth, 'MMMM')
https://docs.mendix.com/refguide/parse-and-format-date-function-calls/#3-formatdatetimeutc
Hope this helps.