@Sunshine thanks, yes that looks like the issue.  The ax6000 does not re establish connection without a restart. Perhaps the restart lost relevant log entries?  I am pretty sure the wan IP address and virgin media DNS was good at the time of the problem (checked locally on the modem) but I will have to wait until the next failure to check on the AX6000.  Or should I change the DNS server settings anyway?  If this problem was introduced by the recent firmware update, is it possible to revert?  (I just accepted the upgrade suggested by the Tether app - is this reliable?)

  @Mutineer 

If the AX6000 still has valid WAN IP address when the issue happens, you could change the DNS server first for monitoring.

According to the release note of the current firmware, it cannot downgrade to previous firmware via the regular process, if it is really necessary, you could try firmware recovery:

How to recover the router when it bricked

  @QRene great, I'll change the DNS as suggested in the post and then see if that makes any difference to stability.  Should I use Google DNS servers instead of those suggested here, or those of Virgin Media?

Firmware recovery looks like a last resort.

Many thanks to you and @sunshine for prompt response.

  @Sunshine yes thanks I'll use the web gui from now on and check logs as first step.  Is there a handy guide to log entries that you could point to?

I think I will try Cloaudflare DNS.  Seems well reviewed in UK.

I spoke too soon. The AX6000 failed to renew an expired lease with the VM hub 3 modem, which appeared to have granted a new lease properly, but the AX6000 connection did not re-establish:

Last entries before the connection died:

2023-09-04 14:01:33 DHCPC INFO [9115] receive ack from server with ip X.X.X.X, options(serverid=Y.Y.Y.Y;lease=467365;subnet=255.255.252.0;router=Z.Z.Z.Z;dns=194.168.4.100 194.168.8.100;)

2023-09-04 14:01:33 DHCPC INFO [9115] send select request with options(cliid=01/.....:;)

2023-09-04 14:01:33 DHCPC INFO [9115] 1/2 lease passed, enter renewing state

Last entries after reboot:

2023-09-04 16:00:12 UPnP INFO [10634] Service start

2023-09-04 16:00:11 UPnP INFO [10634] Service stop

2023-09-04 16:00:11 Remote Management INFO [10364] Service stop

2023-09-04 15:59:59 NAT INFO [9085] IPSEC ALG enabled

2023-09-04 15:59:59 NAT INFO [9085] L2TP ALG enabled

2023-09-04 15:59:59 NAT INFO [9085] PPTP ALG enabled

2023-09-04 15:59:59 NAT INFO [9085] SIP ALG enabled

2023-09-04 15:59:59 NAT INFO [9085] RTSP ALG enabled

2023-09-04 15:59:59 NAT INFO [9085] H323 ALG enabled

2023-09-04 15:59:59 NAT INFO [9085] TFTP ALG enabled

2023-09-04 15:59:59 NAT INFO [9085] FTP ALG enabled

2023-09-04 15:59:59 Traffic Statistics INFO [8349] Clear all stats because time changed

Any ideas? Or is it 'normal' behaviour to fail to re-establish a connection after lease renewal?

This router is still unstable, failing every 2 days, and will not re-establish a connection with the upstream host.  A physical power down / up is needed every time it fails as we also seem to lose hardwire and WiFi client connections to the AX6000 (no access to console or logs).  There is still a bug in the last two releases of the firmware that seems to be causing this. Changing DNS settings seems to have little effect.  Last resort is to revert to factory firmware which was the last time it was stable, or otherwise return this to the retailer with a complaint that it's no longer of usable quality.