Your LimeSurvey version:  Version 3.26.5+210519
Survey theme/template: Custom Vanilla
==================
Hi Team,

Is there any way to send auto reminders to the users? I have found the plugin which is paid but we don not want to use that approach.Please let us know if there is another approach you know.

Thanks in advance!

Of course you can do it by yourself.
You may create a small script in php which queries the database by the API (or directly) and sends a reminder where necessary.
This script you call each day by a cron job. There are many providers for free cron jobs.

Joffm

As Joffm says, if you don't want to pay for the plugin, then you can always create your own plugin. I this is cheaper or better, I don't know.

Here another plugin that might be helpful. You'll need to see if it does what you need:

extensions.sondages.pro/mailing-manageme...n/documentation.html