Changing router results in arprequest: cannot find matching address same config

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HI I am trying to swap out a Cisco 887VA-K9 with a later model 897VA-K9 teh configs are the same this is configured for simple bridging and the PPPoE is managed by OPNSense.

The cisco is simple acting as a FTTC VDSL2 Modem and bridge to OPNSense WAN interface using a point to point config.

In the 887VA-K9 this works perfectly.

In the 897VA-K9 all looks well in the WAN overview page. Connected and IP address etc. But I get very weird issues on the lan with connectivity websites time out cannot run speedtest etc. It's as if there is some kind of routing issue. The log gets constantly spammed with these messages:

arprequest: cannot find matching address

I could not find much information about this in respect of my issue.

I tried rebooting OPNSense in case it was an arp table issue having changed out the router. There is a MAC address set on the WAN interface page that matches the MAC of the WAN Nic in the server I can see this get assigned to the WAN PPPoE connection. This works perfectly when the 887VA-K9 is plugged in.

If I unplug the 897VA-K9 and plug the 887VA-K9 in all bursts into life.

My instinct is it is something to do with the MAC of the 897 vs the 887 but the WAN interface has the same settings for both.

Any help appreciated as we would like to swap these old 887 units out.

Cheers
Spart