Is there a trick to deploying a T3 snapshot onto a T2 instance? Not working for me...

The most likely cause here is that the network drivers (that were automatically installed and enabled for the T3 instance) are not compatible with T2 instances.

You haven't mentioned what operating system you're using so the brute force approach would be:

• Spin up a T2 instance
• Find out what drivers are being used
• Enable (or install and then enable) those drivers on the T3 instance
• Take the snapshot