howto next date expiration mail remainder

Hallo,

I am quite new with WHMCS and it is rather a complex platform to get help by documentation.

I just need to send remainder mail 30,20,2 days before the service gets expired.

I created my template with info about services, ammount and payment details, but despite the cron job configuration no mails are sent.

Note: I do not need to send automated invoice via mail before or after deadline as clients might not renew. They are monthly services. Invoice is generate automatically and sent via mail after date.

Please any advice about howto cope with it ?

Thanks for your attention.

Regards