Can't get SM-900 to be accessible with fastboot on Linux

I'm using Debian GNU/Linux trixie (well, the Devuan equivalent to be exact), and am working on a Samsung SM-P900 Galaxy Note 12.2 Pro tablet. As part of possibly flashing it, I need to make sure the bootloader is unlocked. I have adb and fastboot installed. What I see is:

# adb devices
List of devices attached
5200b47af69a4247	device

# fastboot devices

so, visible with adb, not with fastboot. I found this question: https://android.stackexchange.com/questions/221634/fastboot-connection-problem which said I can't see the device at the same time with both adb and fastboot, and that I should reboot it like so:

adb reboot bootloader

but when I do that - it reboots as usual, and I still see it only with adb. I also tried the following: 1. Booted into recovery mode (using Power + Center-bottom button + Volume Up) 2. Chose "reboot to bootloader" ... that also resulted in a regular boot (showing my homescreen and apps), and device was still invisible with fastboot. What can I do to get my tablet to be fastboot-visible (so I can flash it)?