OPNSense in Bridge Mode - No Internet after an hour or so

[markp]

I have setup OPNSense in bridge mode  running on a mini PC based on:

https://docs.opnsense.org/manual/how-tos/lan_bridge.html

When booted the system starts up and the internet is usable and everyting appears to be working fine, After about an hour or so the internet is lost. I can still login to OPNSense web-gui but I cannot access the internet.

What should I look for in order to resolve this issue? What could be causing it to block internet access?

When it has failed I can SSH onto the device but from the device I cannot ping the wider internet (e.g. ping 8.8.8.8)

If I reboot the device the internet is available again, so it looks like like the interface connected to my ISP supplied router has failed / stopped.