I have installed the auto-lot numbering SuiteApp. But the custom columns on the inventory details are still not showing up even after setting up the additional lot number fields, etc. Am I missing something here?

For what I know, auto lot number does not create any custom fields for inventory details record, perhaps on the item record you can create a lot number sequence and include those additional lot number fields to make it part of the lot number that is generated by SuiteApp.