Does your master socket have 1 or 2 ports at front?
You sure you have a NTE5C master like Linebox NTE5C User instructions ?
The older OR filtered faceplates provided filtered (of the DSL from the voice) extensions, which seems to be your issue, but I'm not sure the NTE5C does.
Is your extension wiring connected to the blue (or white) terminal block behind or the red 1 on back of faceplate?
I think you'll find the extension wiring is connected to the blue (or white) terminal block, which is filtered. For BB at an extension, you need to connect it to the red block.
If your low speed was due to the extension wiring, then altering the wiring so that the broadband appears on it, is going to bring back the low speed problem again.
The best option is to run a separate RJ11 data extension cable from the front RJ11 socket on the NTE5C, to wherever you want the home hub to be located.
I thought the issue was that BB didn't work at all from extension socket in study, not that it was slow.
if your master has 2 sockets then try connecting router with rj11 modem cable to the adsl/vdsl outlet on master and then to hub and post your router stats to see conenction speed and noise margin