This is what I do
To get the file use curl (on Linux, macOS or Windows 10)
curl -v http://localhost/api/calendar/file_name.ics
You will see
< HTTP/1.1 200 OK
< Date: Mon, 22 Jul 2019 17:52:59 GMT
< Server: Apache/2.4.18 (Ubuntu)
< Content-Length: 648
< Content-Type: text/calendar;charset=UTF-8
<
BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
DTSTAMP:20190722T175259Z
SUMMARY:An Event
DTSTART:20190716T131500
DTEND:20190716T191500
END:VEVENT
...etc
You can save the ics file
curl http://localhost/api/calendar/file_name.ics -O
Try opening it in a calendar app like Mozilla Thunderbird, Apple Calendar or Outlook or import into Google Calendar or Outlook 365