Have just installed the 2866ax at another site that also had a 2860n in place & is again a Vodafone FTTC line, used the same settings as at the first site and it got to Showtime quickly - then changed the user name & password to those of the new site, rebooted and connected to Internet within a few seconds. So router is not DoA!
Will return to site 1 & do the same in reverse ie connect 2866, see if it gets a line sync & then change the username/password over to that site's - have printed off the DSL stats at site 2 to identify any differences (Site 2 is a short line on a 40/10 service that gets full throughput - Site 1 is a long line that synchs at only 4.5/1.5)