Changing Router from Cisco 887VA to 897VA results in no wan traffic

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Hi we have recently been trying to replace an old 887VA with a  VDSL Bridge config with a 897VA with essentially the same config.

Opnsense PPPoE connection completes perfectly and we can see the Public IP's are assigned correctly but there is no traffic via the wan port. Also the network starts to behave weirdly. Websites failing etc. We though it was something to do with Arp tables but having rebooted everything. The Cisco Router. The OPNSense box, the central switch when everything comes up all looks good but  no routing of traffic is happening. If we power off the 897VA and plug the 887VA back in all bursts back into life.

The config is the same. It feels like it is a MAC address ARP issue on the wan link like it is trying to send to the old router.

Does anyone have any experience of swapping our a router and the gotchas involved. We just expected this to work as it is upstream of the OPNSense box and the PPPoE connection seems to work just fine.

Help appreciated as we need to retire the 887 as it is out of support and we have a new 897 to replace it.

Cheers
Spart