Without knowing all of the details this gives the impression of a vectoring conflict. I assume your DSLAM uses vectoring, but your firmware for the vdsl-modem does not support vectoring and the DSLAM put you on the fallback profile for non-vectoring capable CPE. At least deutsche telekom, does not use vectoring on the lowest 2.2MHz of the spectrum (otherwise, they would need to switch out all non-vectoring capable ADSL[1|2|2+] gear in the field with vectoring capable VDSL2 gear, probably too expensive) and any modem/CPE that does not negotiate vectoring with the DSLAM will be assigned a profile where it can only use those vectoring-free frequencies, as not to negatively interfere with the vectoring processing in the same binder.
Tl;dr: try a firmware blob that supports vectoring, and talk to your ISP as some DSLAMs need a manual port reconfigure (or similar procedure) to get a line out of the fall-back profile again.