Can you share ISP's name, is it connected through normal LAN cable and also is it needs to be authenticated through username passwd before connecting (pppoe). what is the internet speed? Is it a telephone line connection or through cable?
You need the mighty terminal and wvdial. I haven't used kde recently, so no idea about the gui interface for nwmanager. But, if your lan card is supported, check again with kde network manager for options to set up username and password (in ipv4 tab usually).
first run "ifconfig" to identify the ethernet node (usually eth0 and loopback address). whether the NIC card is supported or not needs to be find out.
Normal procedure is open terminal, run "sudo su" to gain root access, run wvdialconf /etc/wvdial.conf and then you need to set manually the parameters like username/passwd, baudrate etc. Also, if you don't know vi/emacs usage, then run "kdesudo kate /etc/wvdial.conf" or "kdesudo kedit /etc/wvdial.conf" to enter the username and password.
edit (kdesudo kedit filename)
Wvdial - ArchWiki
DialupModemHowto/SetUpDialer - Community Help Wiki