debian stretch loses gateway

[Richard]

After working for a while, the network repeatedly becomes disconnected, with the message connect: Network is unreachable

How to reproduce the problem:

After performing connmanctl and connecting to a WiFi AP, I get

root@beaglebone:~# /sbin/route -nee

Kernel IP routing table

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface    MSS   Window irtt

0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 wlan0    0     0      0

192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 wlan0    0     0      0

192.168.1.1     0.0.0.0         255.255.255.255 UH    0      0        0 wlan0    0     0      0

192.168.6.0     0.0.0.0         255.255.255.252 U     0      0        0 usb1     0     0      0

192.168.7.0     0.0.0.0         255.255.255.252 U     0      0        0 usb0     0     0      0

I then try to do some work -- specifically

root@beaglebone:~# apt update

root@beaglebone:~# apt upgrade bb-node-red-installer

This fails spectacularly, with lots of errors, starting with

Err:1 http://repos.rcn-ee.com/debian stretch/main armhf bone101 armhf 1.1.12-0rcnee1~stretch+20180920

  Temporary failure resolving 'repos.rcn-ee.com'

Looking for the cause of the failure to resolve repos.rcn-ee.com and deb.debian.org (which appears in lots of the other errors), I again run

root@beaglebone:~# /sbin/route -nee

Kernel IP routing table

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface    MSS   Window irtt

192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 wlan0    0     0      0

192.168.6.0     0.0.0.0         255.255.255.252 U     0      0        0 usb1     0     0      0

192.168.7.0     0.0.0.0         255.255.255.252 U     0      0        0 usb0     0     0      0

The gateway has gone missing. Why?

I'm running Debian 9.3 stretch, BeagleBoard.org Debian Image 2018-01-28,  on a Beaglebone Black Wireless.

root@beaglebone:~# uname -a

Linux beaglebone 4.9.78-ti-r94 #1 SMP PREEMPT Fri Jan 26 21:26:24 UTC 2018 armv7l GNU/Linux

I'm mystified, but I assume that there's something that explains this. And even more usefully, something that fixes it.

Thanks,

Richard