When you create your reminders in MemberPress, you should test them to ensure that they work correctly.
This document will help explain how you can test your reminders in MemberPress.
Setting Up Tests
The best way to test if Reminders are working properly is to go to MemberPress > Reminders and set up a new Reminder for “0 days after Signup Abandoned”. Once you have that in place, also enable the Send Signup Abandoned Reminder Email to User setting. Then Publish the reminder to save it.

In addition, ensure your registration pages are configured as two-step registration.
Testing Reminders
After creating the Reminder Email, create a test user by filling in the registration form and submitting it. On the second step, don't complete the payment and leave the page (abandoned cart).
Once you get to that point, wait about an hour to see if the Reminder email goes out. If so, then the Reminders are working as expected.
If you did not receive a Reminder Email during the test, then your WP Cron may not be running correctly on the site. If that's the case, reach out to your host and ask them to check whether there are any issues with WP Cron. Once resolved, try rerunning the test.