I had a feeling DLM was only on VDSL, but that might have changed.
Do you know how old your old modems are? ADSL2+ has been standard for a very long time (>10 years), so chances are it's only the WiFi performance that has improved.
And again... strongly recommend getting to the ETP, disconnecting all internal wiring, and connecting the router and a phone (via filter) directly to the incoming line.