Searching Custom Fields in Zoho CRM

Hello,

I have created a series of custom fields in Zoho CRM in order to add details which are crucial to my business.
Most of these custom fields were added in the Vendors tab.

I noticed these field are not indexed and consequently not searchable by the CRM search engine.

All that can be retrieved are the original fields (even if it seems not all of them. E.g.: Category field)

Is there a way to include custom fields in the search engine searchable items?

Thank you
Roberto