Hi,
You start in interesting job if it's the first time you (try) to build modules
I don't use the "support" methodology ... but I am certainly not so expert than us
It's not easy to shortly write a response with all the detail of operations and all possible error, mistake ... especially when adding cross-compile ... that I have abandoned from years (except for creating a first working environment on a platform or for very limited platform as IoT support for ex. )
I use the "native" method
the steps are (shortly and sorry but no time to enter in detail ... ask support) :
I start at this point without cross-compile (replace by an Ubuntu 14.04 chroot INSIDE the NAS (in this case a 1002T) with the build-essential package and curl for an easy menu approach to configure kernel)
get GPL source, extract it in a safe place (not cleared at boot) with write right
copy the config file in the base of the GPL source rename it .config
run
make oldconfig (validation + generation of basic libs)
then
make menuconfig
... in this menu select the modules you need with M to generate modules
At end (save new config)
generate the module
make path_to/module.ko (or all the modules in a folder ... depends of your request) be attention to dependency between modules
IT'S ABSOLUTELY NOT necessary to rebuild all the kernel and modules to add only one or two ... (so conflict with support recommendation)
and you win hours ... (if you have doubt rebuild all modules but not all kernel)
move the module to the good place ... /usr/local/bin don't appear to be a safe place it's not binary files ... the default place is /lib/modules/KERNEL_VERSION/ but clear at each boot and linked to builtin
... creating a fake APKG can be good (so you can also later load modules automatically at boot)
last insert it
... suggest to don't use modprobre (only rrmod and insmod) to avoid future problem if a true modprobe is delivered in a new ADM or through any APKG (ex. Entware)
... the modprobe provide in Asustor is the castrated Busybox version ... so absolutely NO interest ...
So I can't help you any more ... and suggest you to follow support recommendation to get also help ...
F.Y.I. reason of my choice :
chroot will be done only ONE time and can be used for lot of other usages ...
creating native module is less of 10 mns
No need to have another platform with cross-compile tools to build something (only the NAS and a ssh console)
deferring the action during the night or reducing the priority to don't impact normal usage is in "a" command ...
screen or tmux can be used if you don't want to stay connected (even time is short)
BUT I repeat ... using Asustor solution if it's the first time you run this kind of things ... is better ... because you can have professional help.
Philippe.