@SirPodgio
I would strongly suggest that you do not connect the DSL-AC68U to your line. It is probably has the worst ever VDSL2 modem chip which performs very poorly on Openreach FTTC equipment and causes DLM to severely restrict your line speeds.
This ^^^
If you value your current sync you won't even connect the Asus.
G.INP is banned on these devices so if you're on a Huawei cabinet the sync will be much lower.
It's a horribly unstable modem and should be returned to sender immediately.
As for the sync at the moment.... the only difference between 40Mb and 80Mb products is an artificial cap at 40Mb.
Your line syncs less than 40Mb.
The upgrade has done nothing and really isn't worth it for you.
I'd ask them to downgrade you.
My current bundled NOW Hub Two router is rubbish from this point of view (although general use I have no problems).
It may not be the best router in the world but it syncs at a similar level as my Zyxel 1312-b10a does, and that's the best modem I've tried.
I wouldn't waste money on a different modem to get a higher sync. It's not that simple. Your line doesn't reach 40Mb and a new modem is unlikely to help that.
If I were you I'd go on eBay and see if you can pick up a decent Broadcom based modem-router - eg Zyxel VMG8924-B10A or VMG3925-B10B - they can be had for very reasonable prices. The high quality modem side of these may be able to get you over that magic 40Mbps (no guarantee though).
Have you tried the Now TV Hub 2?
It syncs higher than my Zyxel 8924.