CG3000 Temperature - how to find - and what is acceptable range

I have recently re-arranged my home office, and the best location (ascetically) for my Cable Modem, ReadyNAS, UPS and Desktop is in a cupboard. This has only a 90mm cable access hole at the bottom rear.

All is working fine. Even the WiFi coverage is really good, despite the modem's location.

I have a temperatore monitoring probe in the cupboard, and it shows between 28C and 31C.

The UPS has no monitoring available, but is not overly hot. The desktop CPU cores max out at 60C, but mostly around the mid 40s - so no issue there. The ReadyNAS104 fan is at 800rpm and CPU at 68C, disks about 40C.

The Cable Modem however feels hot to touch. It always was, even if out in the open. 

My question is, how hot is too hot for the CG3000v2? Is there a specified operating temperature range? Are there any temperature sensors available in the CG3000v2?

Thanks

Peter