Index not working in SQL statement containing IN

We had this querry that executes every second. I tried supplying an index on delivery_date_time but it is not using. (FULL TABLE SCAN was executed)
We expect this table to grow by 3.5 million data so querry must be optimized.
Actually the expensive SQL is the inner SELECT statement.
I inserted around 2.6 million data and ran this querry.

Any hints optimizing this querry? probably which index to use?
By the way, the SQL statement is autogenerated by hibernate.

CREATE TABLE PUSH_DELIVERY_MGT (
    DELIVERY_ID                 NUMBER(20) NOT NULL,
    CUST_CON_NUM                VARCHAR2(32) NOT NULL,
    DEVICE_ID                   VARCHAR2(11) NOT NULL,