Minor Salesforce Development Task

We use the basic version of Salesforce (Essentials edition) and have a very simple CRM. On our old CRM system when a staff member added a note against a customer's file it changed the 'last modified date' and worked great. The staff then use a filter to show me all the opportunities that have not had updated notes in the last 30 days. We have moved over to Salesforce but the issue we have is that when we add a note, it does not change the modified date of the opportunity. In the main opportunities module, we need to create a filter that shows all of the accounts where the notes have not been updated within 30 days. It looks like there is no standard way to do this wiothin saleforce. We will likeley nee dto set up a custom trigger. Can someone help us with this?