I live in a Victorian property (plenty internal brickwork and lath & plaster) that has four floors including a basement.  My modem/router is in the basement close to an external wall rather than centrally situated.  This is so I can have a short RJ11 cable connecting to my landline master socket.  
My broadband is supplied through fttc but as my property is some way distant from that cabinet the BEST download speed I can record is about 25Mbps.
I have a standard single phase supply to the property that splits to supply two consumer units (circuit breakers), hence two 'circuits'.  One 'circuit' is protected by RCD, the other is not.  For a good number of years I have been introducing the output of the router into a power socket on the NON-RCD PROTECTED 'circuit' as that's what is nearby.  Together with a number of wifi and LAN adaptors) and the wifi from the router I have covered the property to my satisfaction.
Recently I installed a smart TV into a new location and attached it to an adjacent adaptor using a shielded RJ45 patch cable.  It's hopeless!  The speed fluctuates between 15 and 0 Mbps.  This adaptor is plugged into a socket on the RCD PROTECTED circuit, so to get there the signal has to pass through the RCD circuitry in the other consumer unit.  That would not be the case if the signal were introduced via a RCD PROTECTED socket.  Could this be a reason for the problem?

Also,if I were to take a SECOND patch cable from the router and introduce the signal through another adaptor to the RCD PROTECTED circuit could this provide a solution?