While using my MacBook which connects via wifi the internet speed is sometime terrible. upon checking with speedtest.net it sometime informs me i'm connected via vodafone and not Virgin media. When connect via VM I get between 90-100mbs when vodafone is listed as my service provider it's 0.5-2mbs. This also has happened on my flatmates mobile phone sometimes. Restarting the MacBook sometimes fixes the issue.
Do you have a mobile somewhere that’s previously been used as a wireless hotspot?
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This issue happens not infrequently, what you will find is that a neighbour has Vodafone broadband and is also using Powerline adapters and their's and yours are bridging. It should't happen between supplies but does although the connection quality is generally really bad but sometimes enough for one of your devices to appear on the other persons network and vice versa.
You will ned to check the instructions for your particular model but PL adapters all have a means of encrypting traffic between them so that they can't talk to 'rogue' devices - it might be enough to factory reset all of yours and then repair them.