So adding a iCal url doesn’t seem to work with NextCloud (https://www.calendarlabs.com/ical-calendar/ics/76/US_Holidays.ics). I did some google searching and found this https://help.nextcloud.com/t/subscribe-to-calendar/70758/2. Not sure if this is the problem but it claims the NextCloud calendar cron job isn’t running or crashed.
I verified that the ical url works with Apple Calendar and Thunderbird calendar so It seems like nextcloud. That url is also listed in that bug.