i had also my own struggle with this guide https://github.com/recalbox/recalbox-os/wiki/Load-your-roms-from-a-network-shared-folder-(EN) - very nice but wlan does not work.

But I solved it with a workaround:

try the implementation as shown in version 4.0.1 (mount the nas share before starting emulationstation)
Edit /etc/init.d/S31emulationstation - this way

start)
ifconfig wlan0 up
mount -a
enabled="$systemsetting -command load -key system.es.atstartup"
videoMode="$systemsetting -command load -key system.es.videomode"
if [ "$enabled" != "0" ];then
echo $videoMode | grep -qE ...

at the end of the system start you need then a restart of the S31emulationstation-service.

Edit the /recalbox/share/system/custom.sh which is referenced by /etc/init.d/S99custom
and put there the line

/etc/init.d/S31emulationstation restart

Don't forget an "chmod u+x /recalbox/share/system/custom.sh"
so that it can be executed.

Background: anything in the binary of the emulationstation activates wlan0. But then it is too late for mounting the share of the nas. So when we restart the service "mount -a" can be executed correctly because wlan0 is up. Maybe we need no "ifconfig wlan0 up" because it should be brought up by a configuration-file which is loaded before but sure is sure
this worked for me!