OK all understood,
Don't worry, we don't close topics here on this forum (as a rule!)
Turning now to the use of the router/modem whilst plugged into a non BT master socket.... whilst, as your Dad has found, it can sometimes be made to work this way, but it is strongly recommended that only the master socket be used for this to provide reliable consistent and the best speed performance from the equipment
Using a subsidiary socket is generally a very bad idea, leading to slow downs and inconsistent performance...... whilst making house calls I have from time to time come across customers who were unwilling ( for convenience reasons) to plug into the master socket, citing access /wiring issues that they were unwilling to address, and that's their decision of course, however if they want the best performance possible from their network then they should follow the recommendations.
Talktalk in their information sheets and customer booklets make specific reference to this as do practically all ISPs and manufacturers of modems/routers.
Subsidiary sockets are fine for telephony transmissions but were not generally designed for data carriage, hence the degradation in performance when using them for data. Different modems routers will differ slightly in their performance characteristics when used in the non approved way.
The check using the Bt test socket ( behind the faceplace) mentioned in step 4 in my previous post is designed to rule out internal house wiring and subsidiary socket/wiring issues with the upstairs or other subsidiary sockets throughout the house
If the speed readings are consistently similar when using the BT test socket to those currently being obtained through a subsidiary socket then it has ruled out most potential house wiring/ socket issues "downstream" of the BT master socket.
Let us know ( when you can) how you get on.