You get no index usage because you indexed a column but you are querying it via an expression.
To enable the usage of an index you must not use the index expression (here: the column) in an expression.
From the way your query is written I assume the type of the new date_time column is VARCHAR2 (otherwise you wouldn’t convert it, I guess).
The best solution would be to store the date_time column either as DATE or TIMESTAMP (not VARCHAR2). Then index the column and if you have all that, rewrite your query to use the column directly:
WHERE date_time = TO_TIMESTAMP(‘&from_date’,’dd-mon-rrrr hh24miss’)