Have you tried rebooting Arc Ultra?

Yes uninstalled the arc, sub 4 and app and reinstalled and still the same

Hi ​@Rich666 

Welcome to the Sonos Community!

Sorry to hear about this issue you are having with your Sub 4 going missing from the Sonos app.

I recommend you get in touch with our technical support team who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports - it sounds like your router might not be allowing the Subs connection via the Arc Ultra (DCHP Proxy not working). Wiring the Arc Ultra to the router with an ethernet cable would likely bypass this issue, if indeed it is what the issue is - if you call in, we will be able to confirm it.

I hope this helps.

After alot of time and energy wasted trying to connect my Sub 4 to my Arc Ultra including many factory resets, re-installing App, etc…. I started a chat and they could not solve it.   Then ended up with a live service person and they connected remotely to my system and told me that the hardware used for Wifi Access point from company RUCKUS has many known issues with Sonos...specifically blocking the ability to connect Subs or surround sound speakers wirelessly with the Arc Ultra.   This is a brand new install at my building so the hardware can be no more than one year old.   I wonder how many other Wifi gateways Sonos has problems with?  There are some complicated fixes.   Here is one article: https://support.ruckuswireless.com/articles/000006140  

After multiple times of resetting all the equipment, resetting the router and nothing working. My house lost power for an unrelated issue and after getting the power back on I just happened to notice I had bass again while watching TV. We shall see how long works this time.

​@Gozyr Here’s a list, but the Ruckus isn’t on it: https://support.sonos.com/en/article/incompatible-network-hardware

Surrounds an Subs get their ip-address through the soundbar. Some routers cannot handle this.

Thanks for the info, I have Vodafone Ultrahub router 

I’m curious, what did Sonos support say, when you called in? 

I didn’t contest Sonos as it I back in but it’s disconnected again and once again I can connect it to the app but when trying to connect it to the ultra arc it says connected at first and doesn’t work so guess I’ll be calling them for help

This sounds like an IP issues that can be solved by assign static IP address to both the Ultra and Sub.

But first, power off everything: Sonos speakers, router and phone.

Then begin with the router, power it on and allow it to fully boot. Then do the same with the Ultra and Sub. 
power on your phone and connect to your system.

If everything is back to normal, you have a network/router issue, but assigning static IPs should help.

I’ll give it a go when I get home next week, not sure how to assign an IP but I’m sure there’s a you tube video for it

If you can find a video for your exact router and firmware version it might help, really unnecessary though as it is just going to say click here, type numbers there.

Better to just open the DHCP Settings page on your router and follow the instructions there. You will likely be done before you could find the right video.

When updating software have encountered before,,, but off and on back become normal…