Attempt at diagnostic has killed my broadband

[#1]

I've had an ongoing problem with my FTTC dropping to very slow speeds in the evening, sometimes as low as 2Mbps (normally 47).
I raised a support query and it took PlusNet a week to respond (yes, a week!)
They couldn't see a problem, but sent instructions to test directly from computer to BT modem via PPPoE ... followed the instructions, but it didn't work (kept getting "error 651", and yes I was using correct login credentials).
So reverted to normal ... DSL lamp was flashing for several sequences, but eventually locked on. However, I now have absolutely no internet connection. The router just reports that it's attempting to connect. I've reset everything to no avail.
Tried to call the support line, but after waiting 55 minutes ("estimated 30 minutes") gave up and am using a neighbour's connection.
PlusNet - please get your finger out.

[#2]

Sorry to hear the connection isn't working.
It looks like it was stuck on our end. I've just done a remote session kill and that should work.
Can you try it again now?

[#3]

Yes, it is now working. Thanks.
But why should attempting to set up a test PPPoE connection direct into the modem cause this? There's no way I'll be doing that again.

[#4]

The PPPoE connection won't have caused this.
The last connection didn't close properly so we had to do it manually.
I dont think you'll see that issue again as it is quite rare.