I’ve run into a similar situation with Helpdesk on Odoo Online, and while it’s true that many reminder-related customizations are more straightforward on the On-Premise and Odoo.sh versions, there is a workaround that might help on the Online version.

One approach is to use Odoo's built-in automated actions (also known as "server actions") along with scheduled activities:

What you can try:

Set up Scheduled Activities on tickets with due dates:

You can configure activity types and attach them automatically using automated actions when a ticket is approaching or past its due date.

This way, your agents will get a notification in their "My Activities" dashboard when tickets are overdue.

Use email templates with automated actions:

While limited on Online, you can still set up basic automated actions to trigger an email to the assigned user when certain criteria are met.