If you’re sure it’s not working (upping the bass in Equaliser setting and increasing the Sub output level) you might want to send Sonos a Diagnostic Report to check it’s working/dead: 

Hi Nik,
 

Follow what u said and still doesn't work out.
 

My diagnostic report 91806543.

Hi @Irwan Lim, I wasn’t able to find a diagnostic with that confirmation number I’m afraid.

It may be worth getting in touch with the Support Team, as they’ll be able to take a closer look at your system, and offer a replacement Sub if necessary.

Hi Xander,
 

Thanks alot for your prompt reply. I will get into it. Thank you. 

Hi Xander,

Anyway, I submit another diagnostic report 106227265. Thanks.

Thanks for sending that diagnostic over @Irwan Lim, looks like Sonos is detecting an unsupported extender, which is causing issues for the Sub communicating with the Arc. Would it be possible to either power off the extender, or connect the Arc to the network via Ethernet cable?