What is the use of the "Index Finder Months" in the payment increase agreement template?

Summary:
We expect that the index finder month to looking for the assessment date minus XX month (e,g, -2 month), however, the rate seems didn't change regardless of the index index month setup.

Content (please ensure you mask any confidential information):

Our customer has a requirement to increase annually their contracts, but the index is know at the end of the month and they pay the month in advance, whereby the customer increase the payment in November with the index of September. However, if we select September, Lease increases the payment in the same month of the index.

We tried with "Index Finder Months" -2 in November, but the index didn't change, Lease selected the index of November.