There is a fantastic tool to see and monitor Bandwidth and Network Traffic on Linux.
bmon and it works out of the box.
How to Setup and Use
apt-get install bmon, or in newer versions, you can also just say
apt install bmon.
yum install bmon.
Once you have it installed, you can start the monitor by running
bmon from the command-line.
You will be presented with multiple network interfaces (NICs).
Press the down arrow to navigate away from the Loopback (lo) NIC, and select
eth0 (or whatever your desired NIC is).
You can press
d to enable detailed statistics, or press
i to enable additional information. If your terminal window has enough height, you can press both