For issues subscribing via the buttons, please see the instructions below.
Go to Google Calendar and log in.
Find the "Other calendars" section on the left sidebar. Click the plus icon next to it.
Select From URL from the drop-down menu.
Paste the calendar's URL into the field provided: https://calendar.google.com/calendar/embed?src=f9a0b9a63676221cf6a2f0de0b6191c820d4fac107cbd4366c9d1458c156b358%40group.calendar.google.com
Click the Add calendar button.
The calendar will appear in your list of "Other calendars" and its events will be displayed.
Reference: https://support.google.com/calendar/answer/37100
Open a web browser (like Chrome) and go to Google Calendar's desktop version by searching for it and clicking "Desktop" at the bottom of the mobile view, as the mobile app doesn't support adding from a URL.
Follow the steps for a computer above to add the calendar via the URL: https://calendar.google.com/calendar/embed?src=f9a0b9a63676221cf6a2f0de0b6191c820d4fac107cbd4366c9d1458c156b358%40group.calendar.google.com
After adding, you may need to go to your Google Calendar app settings on your phone to ensure the new calendar is synced and visible.
Reference: https://support.google.com/calendar/answer/37100
Go to your device's Settings, then select Mail, Contacts, Calendars.
Tap Add Account > Other > Add Subscribed Calendar.
Paste the Google Calendar URL (https://calendar.google.com/calendar/embed?src=f9a0b9a63676221cf6a2f0de0b6191c820d4fac107cbd4366c9d1458c156b358%40group.calendar.google.com) into the Server box and tap Next.
Reference: https://support.apple.com/en-us/102301#:~:text=Add%20a%20subscription%20calendar%20on%20your%20Mac
Open the Calendar app.
Tap Calendars at the bottom of the screen.
Tap Add Calendar in the bottom-left corner.
Select Add Subscription Calendar.
Paste the iCal link (https://calendar.google.com/calendar/ical/f9a0b9a63676221cf6a2f0de0b6191c820d4fac107cbd4366c9d1458c156b358%40group.calendar.google.com/public/basic.ics), then tap Subscribe in the top-right corner.
Open the Calendar app.
In the top menu bar, go to File > New Calendar Subscription.
Paste the iCalendar link (https://calendar.google.com/calendar/ical/f9a0b9a63676221cf6a2f0de0b6191c820d4fac107cbd4366c9d1458c156b358%40group.calendar.google.com/public/basic.ics) and click Subscribe.
Give the new calendar a name and choose a color, then click OK.