I found a solution that works as this is not the preferred one.

When I connect a managed switch in between the NTU and ClearOS with a configuration that bridges VLAN6 tagged from one port to VLAN6 untagged on a other I can connect. On the VLAN6 tagged port is the NTU on the untagged ClearOS.

What I see when I also have VLAN1 on the same port as the one the NTU is connected to, so VLAN1 and VLAN6 tagged. It is not possible to connect. It will connect but immediately disconnect.

So what I think what happens is that when I configure VLAN6 on ClearOS it is not tagged right or there is also a other VLAN that still exist on the same WAN interface most likely VLAN1 as this triggers the disconnect from my ISP. When with the managed switch I also put for example VLAN4 and VLAN7 tagged on the same port as VLAN6 tagged there is no issue.

So the question is how do I make sure I only have a well tagged VLAN6 on my WAN adapter?