If you don't use the Master Socket, it is likely to have a significant effect on your Broadband speed.
It depends how you are set up. Do you have an ADSL microfilter before your phone? If you do it is possible. On newer installs the microfilter is embedded in the Master Socket and the extensions cannot access the broadband signal.
Generally your better bet is to leave the router on the master socket and work out how to extend the signal.
I would suggest either the cheap and cheerful Powerline / Homeplug solution, or push the boat out and get something like Google Home.