Hi @ShayEd
Welcome to the Sonos Community!
If you’ve bought the Denon just to be a volume control for your Fives, I recommend you return it for a refund while you still can. Although you can connect source devices to the DVR and then feed a connection from an line-level (unamplified) output of the DVR into the input of a Five, the DVR’s volume control would have no effect on the signal going to the Five - it only effects the amplified output of the DVR that would go to wired, passive speakers. You cannot wire Fives to the DVR as if they were passive speakers, but I think you mentioned that.
Connecting the input of a Five to the input of a DVR would result in nothing happening. You should connect the output of the CD player to the input of either Five with a stereo RCA-3.5mm lead.
A non-video version of a DVR is just a stereo amplifier. It may be my age talking, but I refuse to believe these are no longer available! It doesn’t matter though - a stereo amplifier wouldn’t help you either.
An easier option might just be to buy a voice-assistant capable smart speaker like a Sonos One, Sonos Roam or an Echo Dot from Amazon and use voice commands to lower the volume instead. All three are considerably cheaper than a DVR - not just each, but possibly in total!
Other option might be to keep a cheap/old tablet/phone just for the Sonos app - don’t allow it to lock itself and things should be easier for you. Please keep in mind the minimum requirements for Android and iOS, but older versions can be still be used to control a Sonos system even when they cannot configure it.
I hope this helps.
"Common sense is the collection of prejudices acquired by age eighteen." - Albert Einstein