To refer to an infamous quote, "you're holding it wrong."
There are two ways to get devices into Apple Business Manager. (1) You purchase the device through a DEP capable sales channel (Apple Business Sales (NOT retail) or an Apple Authorized Reseller) and you have exchanged your Org ID (available in ABM) and their Reseller ID (reseller will provide) (or Apple Customer ID if buying from Apple Business Sales). In this case, the purchased devices appear in ABM before you physically receive them so they can participate in automated enrollment. This is helpful when you are shipping direct to the end user for zero-touch deployment.
The second (2) way is to sideload the devices using Apple Configurator. In order to do this, you must use one iPhone (or Mac) as the capture device and then the other units must be ERASED and present Setup Assistant. I strongly encourage reading the documentation on Apple Configurator.
For iPhone: Apple Configurator User Guide for iPhone - Apple Support
For Mac: Apple Configurator User Guide for Mac - Apple Support
Using it on the iPhone is (generally) easier as you can select which MDM you want the device to be assigned. But the key point - the device running Apple Configurator cannot add itself. That device must also be reset and another phone with Configurator on it must be used to side load it (or a Mac with Configurator).
Oh, and keep in mind that the phone will then need to be reset a second time to enroll into your MDM. So a quick cheatsheet:
• Select an acquisition phone and install Configurator
• Sign into your ABM via Configurator and go into settings and opt to share WiFi (assuming your WiFi is shared key not 802.1x auth) and select your server you want it assigned to
• Reset another phone
• As the other phone is presenting Setup Assistant, point the acquisition phone at it and move through the setup assistant.
• Do not allow guided setup from another device
• Once the blue swirling globe appears, point the camera from the acquisition phone at the target phone
• The unit will add to ABM and it should reboot and erase again
• This will allow the newly assigned device to talk to Apple's Activation Server and be directed to your MDM for automated enrollment.
Note, if you are not automatically assigning devices to your prestage policy in your MDM you will need to do that manually. If you don't the device will not enroll because it will be unassigned.
Hope this is helpful. Read the documentation.
