hi 'more input',
based on my limited knowledge of linux
i might see the following possibilities:
1. the ip address assigned by your isp (xxx.xxx.xxx.188) is used for your PPPoE and the other interface of the NB1400 will be free for you to assign.
2. in case the other interface )ethernet) has preset ip address, you can try connecting directly from your computer to the ethernet interface and sniff the transmitted package using 'arp' command. Normally if the device is connected directly and it has assigned ip address, the mac address will be captured by your computer's network interface arp-cache. This will be shown using the 'arp' command above. Once you know the ethernet's ip address (and normally is 192.168.x.x), then you can assign your computer's ip address with the ip address in the same range and subnets (normally 255.255.255.0)