I had a play around with my HG659 this evening, and it was reasonably straight forward to get it to act just as an ATA sitting behind my third party router.
To do this, I disabled VLAN tagging on the WAN, removed all connections from it other than the phone service, then connected it to the network via WAN. It also makes sense to disable wireless if you're using it this way.
I just got UFB installed today with Vodafone as a provider. Would you be able to give me a few more details as to how you went about the Huawei as an ATA? I have admin access to the Huawei but the settings seem somewhat counter-intuitive (to me at least!).
Edit: Never mind, after a bit of trial and error I got it working. I'm not quite sure I could describe the steps I went through but if anyone needs to know just pm me and I'll have a go at working out the steps in reverse :)
Yes, the settings are set out a bit differently to what you would normally expect. Happy to write a quick step by step if anyone needs it.