Hey, I have a lot of appreciation for the infrastructure you maintain so don’t take this the wrong way: you’re very confidently overcomplicating this one all over the thread. People run nominal 12v stuff direct from battery all the time and it’s fine. It’s the norm in RVs, boats, and off-grid housing. There’s usually quite a bit of wiggle room. The router I’m using right now won’t actually cut out until ~8v and sees almost 16 during my ancient lead acid battery’s eq cycles. It’s completely fine and has been for years. Not industrial or automotive grade by any means, a low-end Netgear from the thrift store.

You also state “while there is DC power the battery in a UPS isn’t used”. Assuming you mean AC that’s still not quite correct, it’s always being charged at least a little and tapping it for DC is perfectly fine.

That’s my advice to op, and a setup I’ve run before: replace the 12v battery attached to their existing UPS and wire the router/any other 12v devices right to it. A UPS is just a nicely packaged battery charger/inverter/transfer switch. Wiring them to larger/cheaper batteries that don’t happen to fit their casings is also entirely viable, as is running either of their outputs to a load panel. Sky’s the limit :)