Actually,
He told you the powerline adapters were "frazzled" and it turns out they were because you were trying to make them operate in conditions that they aren't made to work in.
Basically, every different join or point of connection, be it in the power network, copper ADSL/PSTN network, NBN Fibre, or indeed any type of electrical/data network, is like a roadblock for the signal travelling through, and in the case of your powerboards, those roadblocks slow you down quite a bit.
But your final advice is definitely spot-on, better to make sure its not a problem with your own set-up before being slugged with a fee for a Telstra techie to come out and tell you it has nothing to do with the line/service to your premise.