After the installation of VDSL 4 years ago, I would face an issue of my line syncing at low upload speeds, after power failure. I then restarted my modem a couple of times for it resync at upload speeds assigned in the package. It used to be 8mb then 12mb and now it's 18mb. Download speeds were usually fine, I don't remember any issue with it other than issue with FTTC equipment or backend fiber equipment.
Few weeks ago, I went in to modem DSL settings and disabled all profiles other than VDSL and profile 17a and 30a. Since then, my line syncs at full 18mb upload speed. Hopefully I have solved the issue of my line syncing at low upload speeds for whatever reason.
Outside UK