Some TVs, even though they might have an ARC port, may not support 5.1 audio output over the HDMI ARC port or the Optical port.
If your TV was manufactured in or before 2015, you may want to check with your manufacturer to see if your TV will output 5.1 surround sound.
You can also take a look at the Rtings.com supported Passthrough list to see if your TV is 5.1 complaint.