1. Identify mount point e.g. via /etc/fstab
2. Find systemd unit (see below)
3. Add the found mount-unit to the After= statement in the *.service file  (see below)

systemctl list-units | grep '/path/to/mount' | awk '{ print $1 }'
 path-to-mount.mount

vi  
/etc/systemd/system/php-fpm.service 

[Unit]
 Description=The PHP FastCGI Process Manager
 After=syslog.target network.target path-to-mount.mount