An authentication issue, which two of us suspect at least is fixable remotely, so entirely possible the engineer if just testing for VDSL signal would see no problems and find nothing to fix.
Two stages to a working connection
1. Getting sync at ADSL or VDSL level and only then can point (2) start
2. Sending authentication details and getting a successful reply before everything times out.
This is why we have asked what is plugged into where, since if the DSL port of the HH5 is plugged into the Openreach VDSL socket on the faceplate we should be seeing line speed data in the HH5, the same as the engineers are suggesting.