Think I've figured it out - when it said to create a subnet inside "your" VPC network, the mistake I made was to create a new one, when what I should have done was to use the one that is already there, called 'default'.

I followed the steps (including my own extra one added onto the end for creating a connector) using the 'default' network, and all worked fine, and traffic does indeed use the assigned static IP (which can be verified by a test service that reports the result from calling https://api.ipify.org.