I was unable to plug it straight into the ONT box as it is a PPPoE connection and it needs username and password authentication. Couldn’t see that option in FreePBX or Debian but I do have Webmin installed so perhaps there’s something there.
I have tested with an unmanaged switch, same result.
I have now tested on a completely different network, fresh install of FreePBX and still the same result.
Any ideas ?