You can read GPS data from the RUT955 via Modbus by configuring the router to be both a Modbus slave and master simultaneously. This way, the RUT955 will query GPS information from itself via Modbus requests. Then, you can send this data via Data to Server using MQTT. In this case, GPS data is sent via MQTT.
Please let me know if this method is not suitable for your needs and what exactly you are looking for.
There are also options for sending GPS data here. However, since you are interested in using MQTT and if the above methods are not viable, you can create a custom script to send GPS data. You can obtain GPS information using the 'gpsctl' command. Type 'gpsctl --help' to see all available options.
Next, take the GPS data obtained from the 'gpsctl' command and send it directly via mosquitto. We have some information on this here, but you can also find more resources online. The message would include the GPS data obtained from the 'gpsctl' commands.
Additionally, you can explore 'crontab' to configure your device to execute custom scripts periodically at defined intervals. More information on crontab can be found here.