I have unbanned my new 3ds using sysdowngrader 11.4 to 11.3, that's all I did oO (a9hl).
I try with my second new 3ds but the downgrade fail (main.cpp:181) boot9strap on this device...
Seems so long, just download the right 11.3 firmware for your 3ds (with 3dnus, it's multiple cia), put it on updates folder in root of your sdcard, use sysupdater > downgrade, and it's gone.