Seems like network namespace isn't enabled in the official firmware release. Since I haven't been able to set up the necessary environment for building a firmware with the kernel option enabled, I will try out vrf for now.
Huh? It's just there. You may need ip-full to use namespaces...
root@cpe:~# ip netns add testns
root@cpe:~# ip netns list
testns
root@cpe:~# cat /etc/banner
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
-----------------------------------------------------
OpenWrt 23.05.5, r24106-10cc5fcd00
-----------------------------------------------------