How far is the Amp from the Arc? Try moving the Amp closer to the Arc. Also try unplugging the Arc and Amp from power for a couple of minutes and reboot your router.

The Amp is in the basement one floor below and probably 20-25 feet away from the Arc.  

Are you suggesting to move the Amp closer on a permanent basis or just for setup purposes?  It would be difficult to move the Amp on a permanent basis given that the speaker wire is run to where the Amp is located.

To work as surrounds, the Amp needs to communicate with the Arc’s private 5GHz radio. To test, move the Amp to the same room as the Arc and set it up as surrounds. If this works, then you know the Amp needs to be permanently placed closer to the Arc.

Yikes, OK that could quite possibly be the issue.  Unfortunately if it won’t work in its current location I think it might be a showstopper - the speaker wires are existing from an old integrated home theatre system so there’s not really an option to move the existing wiring without cutting into ceilings and walls.

Is there any other way around or if they can’t communicate via this 5Ghz channel we’re out of luck?

There is no other way. You need to move the speaker wires so they terminate in the same room as the Arc OR use different Sonos speakers as surrounds with the Arc.

Is the Amp in a cabinet in the basement? Have you tried moving the Amp to a different spot in the basement? The goal is to minimize any kind of interference between the Arc and Amp.

The Amp is in a steel network rack - We’ll try moving it outside of the cabinet.  That’s a great place to start.  Thank you.

No guarantee, but you may want to try wiring both to the router, and let Sonos find the secondary path.