Custom bash script at startup doesn't work (BIG-IP VM 17.1.1)

The way that article is organized may lead to some confusion, because there are two initial procedures at the top that gives instruction to create a script and set it executable, but in two slightly different ways depending on version. Then below the "old version" procedure there is another one for "Enable the BIG-IP or BIG-IQ system to run the script". If you missed the last part, it would lead to the behavior you described.