Is that 15Mbps you are getting connecting wired or wirelessly to your Home Hub? Also, is the Home Hub plugged into the master socket or an extension socket?
In relation to that, please can you find the connection stats from your Home Hub. IIRC in Troubleshooting > Helpdesk. (You need to be logged into the router GUI). It may be necessary to do a couple of other checks once we get the answers to these questions
Basically what I'm saying is we need to make sure you are getting the best possible connection speed in the first place. Poor speeds are very often down to not optimising the home wiring.
Once that is done, possibly an ethernet run inside the house to a socket convenient to run a single cable to the office inside wall, then connect the computer to that. Adding 20 metres or more to fit a phone socket in the office for it is the worst possible solution at the sort of speed you seem to have.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. Sync 57825/13835kbps @ 600m. - BQM
Edited by RobertoS (Wed 07-Sep-16 12:42:25)