The serial login shell is disabled
The serial interface is enabled

pi@raspberrypi:/sys/dev/char/4:64/device/driver $ echo -n "20215040.serial" | sudo tee -a unbind

echo -n "<what to enter here?>" | sudo tee -a bind

echo -n "/sys/devices/platform/serial8250/tty:ttyS0" | sudo tee -a bind

tee: bind: No such device