Hi Amintej, glad to hear it's useful. 2 things I can think to say in response:
1. If using the template I did, you will need internet connectivity from the BIG-IP, at least at the time of the template deployment. This is because the template includes instructions to download certain required packages at the time of deployment (like the HA iApp), as well as do things like license the device. If you don't have this connectivity, you'll need to set this up manually (out of scope of this article). If using the template I have used, it actually needs internet access through both the mgmt interface and the ext interface.
2. The API call that is made at time of failover requires access to the internet from the BIG-IP. So this automatic update of the AWS route won't work without it. However, you could still make this API call yourself. You could write a script that updates the AWS route, along with any other things you need to do in case one of your AZ's becomes unreachable, for example, you may have application-level settings within your app to update in a real failover/disaster.
So you could get by if your BIG-IP devices cannot reach the Internet. You'd need to manually deploy them and set them up (license, etc), and then script the failover scenario (updating AWS routes) yourself. Let me know if this helps!