In DigiCal, we don't change the calendar visibility anywhere other than on the calendars to display screen(see screenshot on step 3) or the hamburger menu that appears from the left of the screen.
The calendar sync and visibility status is shared with all the apps on the phone or tablet. To check if the calendar exists and sync is enabled try opening the stock calendar or the official Google Calendar app and seeing if your events show up.
If it's visible try checking if the calendar is synced in DigiCal and set to display. Using the calendar selection screen, scroll down to the end, and click the "Calendars to sync" button. Please exit and restart the calendar selection screen when you changed any sync settings. Click here if you would like to read the full tutorial.
If the calendar is not visible on the "Calendars to sync" and the native apps, check if sync is enabled for the account. Go to the system settings -> accounts, and open the account details page. Ensure that calendar sync is enabled, and check when the last sync happened. Click here for the full tutorial.
If the above steps do not fix the issues, you can try resetting the calendar storage. Go to the system settings -> apps, find the native calendar app (Google Calendar, S-Planner, etc), open its details page and then force stop the app first, and clear its data. *Note that this will remove all local, unsynchronized changes, including local calendars and its events!
Open Google Calendar or DigiCal and refresh using the menu option to sync calendars and events from the server. This may take a while.