If you're starting off with a Sky branded router, then it's generally recommended to flash it with the Netgear firmware first, as described in the link you included above. After that, you might be able to upgrade it to the DGTeam firmware using the web interface, but it might fail and leave you with a pretty brick. So it's recommended to use the same procedure as before. Actually the best option if you have access to a Linux machine is to use the nftp utility, as described on the DGTeam site.