Nest Protect (battery-only version) will not stay connected

Possible a ping ponging behavior maybe happening with the one battery only device. We saw this with NEST thermostat controllers that were only connected to there internal battery source, would after a period of time, get a low battery notice. This happened due to the NEST seeing many signals and not being able too or not staying locked on to one particular signal, even a close signal. Would begin to search and search and search. This search function caused the battery to drain and would have to be re-charged. I found that for one controller, even while the RBR was close, the NEST would not lock on. Farther distances seems to help. Some of this was on NG, the other portion was on NEST to help fix how there devices locked on and stayed locked on. One fix was to just power connect the controller to a power source so the battery became a non issue. 

Does this happen with the RBS turned OFF? Try with the RBS turned OFF and see if the device keeps connected.

What channel configurations are currently set on the router?

Is Mixed Mode WPA2 and WPA3 enabled on the RBR?

How many RBS are deployed? 
What is the size of your home? Sq Ft?
What is the distance between the router and 📡 satellite(s)? 30 feet or more is recommended in between RBR📡 and RBS🛰️ to begin with depending upon building materials when wired or wirelessly connected.
https://kb.netgear.com/31029/Where-should-I-place-my-Orbi-satellite ‌‌🛰

Have you tried connecting this one NEXT device to the IoT network on 2.4Ghz only and WPA2 and AES only set here?

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Try disabling the following and see:
Armor, Smart Parental Controls or Circle, Traffic Meter.