Sync / profile issues with non-Openreach modem

[#1]

Hi all,
I've got a case open with support at the moment where it looks like the Plusnet systems don't receive line speed updates from customer equipment if it's not a standard Openreach modem. I've been using an Asus DSL-AC68U for about a year with BT and never had this issue; my IP profile (from speedtest.btwholesale.com) would always update to follow my sync speeds albeit with a few kb less.
Since moving to Plusnet - I've noticed that if the line disconnects (only ever due to me turning the power off for DIY) and comes back at a different speed then the IP profile shown will be quite a way off from the actual sync speed which, as I've read, shouldn't be the case at all. My line is very stable, very few errors and Openreach have been out three times now to fix 'faults' and each time they've said the line is as good as it can be expected to be so I'm pretty convinced there is no physical fault here.
What I'm curious to know is if anyone else is using this modem and has had a similar experience (and if you found a solution, please share  8)). I've tried one of each of the Openreach standard modems and while these tend to trigger a profile change more regularly, it's still not 100% consistent. The Asus router meets the BT requirements for connection to it's network (as per here http://www.btplc.com/sinet/SINs/pdf/498v7p1.pdf - not a light read) so it's not that it's third party and unsupported but maybe it doesn't support something that Plusnet rely on to update their records when a reconnection occurs? The Asus doesn't support TR-069 / CWMP but according to the SIN document that's not what it should be used for anyway.
TL;DR - does anyone use an Asus DSL-AC68U with Plusnet and find that their IP profile isn't the same as their line sync speeds
Thanks,
Stu

[#2]

Quote

Plusnet systems don't receive line speed updates from customer equipment if it's not a standard Openreach modem.

We don't receive updates directly to our systems from any customer equipment for this. The connection profile we have is updated from BTWholesale based on regular delta reports sent to us which contain the profile that we match our connection profile to.
The IP profile is controlled by the DLM equipment at the cab/exchange. This should match closely with the sync rate and if it doesn't then it needs us to report to BTWholesale for investigation.

[#3]

Does it actually say anywhere that the DSL-AC68U has passed the testing for conformance described in the SIN 498 document?
There was a previous thread about the same Asus model: https://community.plus.net/forum/index.php?topic=145976.0

[#4]

Hi ejs - thanks for the pointer. I had a search for similar issues a few nights ago but this didn't come up. I don't think devices have to be certified per se, but they have to meet the requirements of BT / Openreach to be connected to the network otherwise they reserve the right to remove it, send you to prison etc.
Chris - I understand that but it doesn't explain why the sync speed for my two devices are almost identical yet the IP profile (appearing to be the bottleneck for my connection) gets increased to what it should be when Openreach equipment is attached to the line.  If it's derived through the DLM equipment there must be something to drive that decision. If I were experiencing stability or speed problems with the Asus I would be inclined to agree that the Asus is probably at fault. But when it was working fine with BT directly for around 2 years talking to what I imagine is the same DLM  equipment, why would it change when Plusnet are providing the service behind that same equipment?