Hello,
Apologies for the delayed response.
If you are looking to install 3-rd party OPKG packages, please take a look at this post here.
When it comes to using MQTT from the CLI, I would suggest using mosquitto client:
opkg -e /etc/opkg/openwrt/distfeeds.conf update
opkg -e /etc/opkg/openwrt/distfeeds.conf install mosquitto-client-ssl
Then, you should be able to use mosquitto_sub and mosquitto_pub to subcribe and publish, respectively. For more information, I would recommend visiting mosquitto manual page here.
Kind Regards,