Edit... You posted while I was typing my reply, so whatever was wrong has corrected itself, maybe the line profile updated to the correct value when the line dropped overnight. You should ask Plusnet for a DLM reset to remove the banding, you could get the full 55Mbs sync if that was done,😀
@longedge Your router stats suggest that you are banded at 49Mbps with a high downstream SNRM of 11dB and the fact that this doesn't change when you reboot the router confirms it. You are correct in stating it is loss of sync on a connection that causes that DLM to take action. DLM would know nothing about the drops in PPPoE connection.
It looks like you may for some reason have been put on a static IP, you need to check the value of your Current Line Speed on the Broadband tab in your member centre account. This is not an actual speed but a profile which is supposed to track the BT Wholesale IP profile and is known to sometimes get stuck at a lower speed. It generally only affects users with a static IP but recently I have recently heard mention of a "Workplace Profile" that can restrict users' speeds, if this is the same thing I'm not sure, if you get no response from forum staff it may be worth calling customer service.
A profile of around 22Mbps is one that seems to occur frequently, possibly because it is the maximum value for an ADSL2+ circuit, why this would happen on FTTC is a mystery.🤔
