No, you are unable to carry forward your unused free ATM withdrawal quota to the following month.
The free quota of SGD$400 for the month is completely unused or only partially used, it would simply reset at the start of the month.
Example 1: In the month of June, with no ATM withdrawals made, the SGD$400 free ATM withdrawal quota will lapse at the end of the month and the quota would reset to SGD$400 on the 1st of July.
Example 2: In the month of June, when an ATM withdrawal of SGD $10 was made, the remaining free quota of SGD $390 will lapse at the end of the month and the quota would reset to SGD $400 on the 1st of July.
*This will take effect on 15 June 2023.