Hi Zak88lx,
It's worth pointing out that the directions you provided in the original post are for macOS and not an iPad, as there would be no option to click File to subscribe on an iPadOS device.
The article Use iCloud calendar subscriptions outlines how to enroll in subscribed calendars but only offers directions for Macs. Specifically:
"If you have a calendar subscription on your Mac that you want to see on your iPhone, iPad, or iPod touch, you can add it to iCloud.
- Open Calendar.
- Click the Calendars button in the upper-left hand corner of the window to see your list of calendars.
- Select the subscribed calendar from the list.
- In the menu bar, choose Edit > Get Info.
- Choose iCloud from the Location menu, then click OK."
If you have a Mac try those steps. If not, Set up multiple calendars on iPad offers this process you may try:
"Set up a calendar
- Tap at the top left, then tap Add Calendar at the bottom of the screen.
- Do one of the following:...
- Subscribe to an external read-only calendar: Tap Add Subscription Calendar, enter the URL of the .ics file you want to subscribe to (and any other required server information), then tap Subscribe."
If still not working, consult whoever produced or distributes this calendar to verify compatibility with iPadOS or to seek additional assistance on that subscription.
Regards.