Comment 12 for bug 1580146

So from @Andrea's latest pastebin:

 - NM shows both modem and wlan0 as connected

 - syslog shows one DHCP renewal period

 - a do-add-ip4-address error is logged, however the IP address is still configured ( ip addr show confirms this ); need to investigate whether the error log message is correct, and possible adjust the text of the message

 - dnsmasq has four nameservers configured ( wlan0: ipv4 & ipv6; modem: two ipv4 )

 - the internet is reachable, as 8.8.8.8 can be pinged

 - DNS lookups fail ( ping google.com responds with 'unknown host' )

 - the DHCP lease interval appears to be just shy of 10m ( 571s )

 - the problem seems to occur after the lease renewal

 - the device may have been asleep between the time the WiFi connection was working and the failure