On Mobile Devices (iOS - iPhone 6) the subscribe button does not appear unless you rotate your phone to landscape mode. We need the subscribe button available regardless of orientation.
I was told this was a "Bug" and that it would be updated if I added this via "Get Help" so I did that. Then Support said this is an "Idea" so I'm adding this here as well.
Example of Missing subscribe button attached. (Yes, "Enable Calendar Subscriptions" is checked under Calendar Settings.)