Hello BananaHackers,
I found a method to get fastboot mode on the Nokia 8110 4G.
I noticed that the boot loader starts fastboot mode if it cannot boot from a bootimage.
Enter the following commands in the adb root shell:
echo 0 > /dev/block/bootdevice/by-name/recovery
reboot recovery
This corrupts the recovery bootimage and makes the boot loader boot from it.
The phone shows the splash screen and does nothing.
But it is in fastboot mode. Try "fastboot reboot" on your computer with the phone connected. The phone should reboot.