The ISP can't, yet, perform a DLM reset without an Openreach engineer. Either they were fobbing you off or they have access to resetting the DLM caution counter now as per the latest Openreach announcement yesterday (though I have doubts).
Some people I believe have had success in removing banding by applying a manual banding of their own for a while (a week or two perhaps). This can be done with modems with a Broadcom chipset typically via xdslcmd, see the sticky about speed capping at the top of this forum category.