Add external calendar to Google Calendar

I think the Google Calendar app for Android doesn't support that functionality.

From a computer:

1. I open a Google Calendar, for instance this one


2. I click on the button.


3. That calendar gets added to my Google Calendar.

From an Android phone:

1. I open a Google Calendar, for instance this one


2. I click on the button.


3. The Google Calendar app opens and displays the message "Calendar couldn't be added".

According to the response from a Google representative, "there isn't an option to add external calendars to the Google Calendar app".