When ‘bonded’ to the Arc, the Sub is no longer directly connected to your router, it instead connects to a 5Ghz ‘hidden’ network created by the Arc.
A couple of things to confirm. First, make sure your Arc is connected to your 2.4Ghz network, or wired with an Ethernet cable. You’ve checked that both have their WiFi turned on, that’s great. The Sub should, through the connection to your Arc, be receiving a proxy IP address, so it should be showing up in your router’s DHCP page.
You don’t specifically say, but I’m assuming your Sub is not working? Or is it working, but just not showing properly in your router?
Please stop factory resetting everything. That should only be done at instruction of Sonos reps, and for most issues, just masks the problem, and doesn’t actually fix it.
Finally, if all the above doesn’t work, I’d submit a system diagnostic, and call Sonos Support to discuss it.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.