Trying to improve my line speed

I upgraded my Plusnet account to Extra last year, and after a visit from an Openreach engineer (something had been misconnected in the exchange, and the 'upgrade' had killed broadband completely) my line speed jumped from the 3.something Mbits I'd been getting before the upgrade to 7.something - it's varied a little occasionally, but generally been very stable, with negligible dropouts. The Openreach man told me that 12 Mbits was available on the wires coming into the house, and my router was probably syncing at a lower speed because of noise picked up in the numerous phone extensions around the house (built in when the house was new, 10 years ago). He thought I would do better if I fitted a filtered faceplate and connected the router there. I didn't want to do that at the time because it would have meant threading cat 5 cable through too many walls.

I've now installed Solwise HomePlugs to get around that problem, fitted a filtered faceplate, and connected the router there. The sync speed increased immediately to 8.0 Mbits, but no more, so I checked the router settings and found I'd left the modulation on G.DMT. After re-setting to Multimode it has re-connected at 8.128 Mbits, but still G.DMT. The SNR margin is 6.0db and the loop attenuation 22.0db.

The exchange is 21CN enabled (as I assumed with 12 Mbits on the line) and I've tried setting my modulation to ADSL2+ Auto, but the router won't sync.

Does this indicate that the router in the exchange is set to G.DMT, and needs to be reset to enable ADSL2+?