How to install mosquitto_sub on Teltonika RUTX/TRB (OpenWRT)

You can just install mosquitto_sub on OpenWRT routers such as on the Teltonika RUTX10/RUTX11 or TRB series using opkg:

opkg update
opkg install mosquitto-client

In case you see error messages like

* check_data_file_clashes: Package libmosquitto-nossl wants to install file /usr/lib/libmosquitto.so
       But that file is already provided by package  * libmosquitto-ssl
* check_data_file_clashes: Package libmosquitto-nossl wants to install file /usr/lib/libmosquitto.so.1
       But that file is already provided by package  * libmosquitto-ssl
* opkg_install_cmd: Cannot install package mosquitto-client.

install mosquitto-client-ssl instead:

opkg install mosquitto-client-ssl

Tested on firmware TRB1_R_00.07.02.6.