Edit the syslinux.cfg file in the LIBREELEC partition on USB/SD boot media and add ssh to the boot params line. This will force the daemon to start on boot. However if this is a clean LE12.x install, something is wrong for the add-on to be faulting or not running and SSH access probably won't solve that, and I'd recommend retesting with a current LE13 nightly first.
The night build also fails to open the LibreELEC configuration; I edited the file, but it ignores it.