I have to add that I'm using only ipv4 to prevent the ipv6 to fail and I sometimes I encounter the same problem than you, the wan interface fails and I have no connection. I power off the router for 5 mins and then I have a new ip. So ipv4 also fails after a while.
I think that opening an issue in the repo maybe helps to give visibility to this problem? At least we are here four or five people, with AX3600, and all with DIGI with the problem.
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Sure!
I just didn't have time to compose the ticket. It would be nice to include a serial log with timestamps.
This doesn't look like the issue discussed in this topic.
Your log just shows a disconnection followed by no servers responding (something down between the router and the servers).
I don't have serial connection 
What is clear is that after a sysupgrade, all the people from this thread is suffering the problem.