Connect it via USB, depend on which USB you have connected the telemetry you are going to need to change /opt/ros/kinetic/setup-mavros-bridge-serial2.bash script and change the following line to the USB that you are using.
rosrun mavros gcs_bridge __name:="gcs_bridge_serial2" _gcs_url:="serial:///dev/ttyUSB0"
On /dev/ you can see the USB available.
After that make the first two commands that I told you to execute:
sudo systemctl enable mavros-serial2.service
sudo systemctl start mavros-serial2.service
You will see an orange light slowly blinking in the telemetry, now if you go to APM planner > Communication > add link > serial.
Connect. After that it should start blinking faster.
If you have some problem you can restart the service or check its status:
sudo systemctl status -l mavros-serial2.service
and also if it's necessary you can restart the apm service
sudo systemctl restart apm