C7100V – Nighthawk AC1900 WiFi Cable Modem Router With Battery PB022

Yes it's been 5 years, but I wonder what version of firmware you were using with this modem?  I purchased a used C7100V and it had 

V2.01.45 firmware.  Maybe this was a bug that was addressed by the time V2.01.45 came around?

Also, I'm wondering what happened to your voice service when the battery kicked in?  I notice that the PB022 battery description for the C7100V on pg 12 of the manual and reiterated at

https://kb.netgear.com/000038533/How-do-I-install-the-optional-battery-in-my-C7100v-cable-modem-voice-router  

only talk about how long it will maintain VOICE connectivity: "The battery can give you five hours of additional talk time and eight hours of idle time for voice service during a power outage."

Maybe that's a clue that it is only intended to maintain the telephone connection during a power outage and the modem is purposely being put to sleep to minimize power usage?