192.168.xxx.xxx is private range..it can't be your public IP.....every provider will give you proper IPv4 .. in your case it is just behind NAT .You just need to check your public IP and also since it is behind NAT most probably it is not port forwarded by default and there are no firewall rules for traffic flow.
So you'll have to do 2 things,first edit firewall and open ports plesk needs to operate(there is list..google "plesk ports")..after that you will properly route external ports to internal(portforwarding).It depend from providers how they handle this...Windows azure has endpoints and firewal and forwarding is done there in one configuration ,others provide two separete configuration..one for firewal second for port forwarding....Any way you must have at least 1 dedicated public IP to operate Plesk or any hosting control panel.
So whatever intrface your provider offer to menage server,look for network configuration and configure network,I don't see how any provider can sell dedicated or virtual servers without at least one dedicated IP...well there is option to have more servers on one IP and than allocate certain range of ports to every of them...but I don't know even one paid provider who does this...no one would buy server with only range of non-provileged ports allcated on IPv4