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Mine is definitely related to ISP IP changes. I lost all internet again this morning, but could access the MiFi device via Luci and ssh. When I checked the logs, the device had been renewing the same IP address from the 4G provider for the last 7 days without incident. Then attempted another renewal in the middle of the night and was unsuccessful. Its next renewal attempt received a different IP address, and from that point forward, the device did not have a connection out to the internet - the router itself could not ping hostnames or IP addresses over the WAN (nor could any of the clients). Oddly enough, it could ping the ISP gateway IP without an issue.

Performing an etc/init.d/network reset restored the internet connection to both the MiFi and its clients (and it retained the same IP that it had received in the middle of the night when the internet connection was lost).

If it were just an issue with DNS resolution, there would be more failure points to check, but since IP traffic is also not going through, it seems like perhaps a routing table issue? I’ll have to remember next time the problem occurs to check whether the default route is perhaps not updating when the IP switches over.