I wonder it is correct.

I reminded my customers of their late payment was delayed.

I reminded my customers about that their payment was three months overdue.

I wonder both of them are correct.

I reminded my customers of their late payment .

I reminded my customers that their payment was three months overdue .

Good effort - Ian

I wonder it is correct.

I reminded my customers THAT their payment was delayed.

I reminded my customers THAT their payment was three months overdue.

OR
I reminded my customers OF their payment THAT was three months overdue.

I wonder both of them are correct. - SEE CORRECTIONS IN CAPITALS

I wonder it is correct.

I reminded my customers of their payment was delayed.

I reminded my customers about their payment was three months overdue.

I wonder both of them are correct.

I reminded my customers that their payment was three months overdue

OR

I reminded my customers about their payment, which was three months overdue

I AM A NATIVE ENGLISH SPEAKER AND THIS IS HOW PEOPLE IN ENGLAND SAY IT:

I wonder it is correct.

I WONDER IF IT IS CORRECT

I reminded my customers of their payment was delayed.

I REMINDED MY CUSTOMERS THAT THEIR PAYMENT WAS DELAYED

I reminded my customers about their payment was three months overdue.
I REMINDED MY CUSTOMERS THAT THEIR PAYMENT WAS THREE MONTHS OVERDUE
I wonder both of them are correct.