I am attaching some screenshots to clarify:
As you can see, there is a 'Back' button in the first one as you go from 'My Settings & Devices' to 'Add another device', but missing a 'Back' button when you go back from 'Enter your phone number' or whatever step after 'What type of device are you adding'.
There are also no 'My settings & Devices' link in the side bar for my Device Management Portal like in your origin screenshot. (!!)
There is case such as a user just navigate around for discovering the portal, and also want to have a way to go back conveniently, and not really want to add a device. The 'Back' button should not be missing when going back, and should allow user to land on their 'My Settings & Devices' just similarly when they first go from 'My Settings & Devices' to 'Add another device'.