The simplest way to solve this problem is to use a broa band modem router to
connect to the internet, rather than the USB device supplied by the ISP. With this setup the router takes care of assigning all the network information and that should avoid the problems of more than one device having the same address.
It may be that the router you are using can work in this way. See ifthere is a single socket at the back marked Wan (for wide area network, the internet as far as we are concerned here) that is too small for a normal network cable to fit in. This socket should accept the phone cable that plugs into the back of the USB modem. If the router does not include a modem the Wan socket will be exactly the same as the other four sockets. Thankfully a broadband router is not expensive.
Once installed, you need to tell the router to give out network addresses for all the devices on the internet. This is known as enabling the DHCP server. Most routers do this automatically, but check the options in the web interface to make sure. The details of how to access this vary between routers so check the quick-start guide for more information. The router will make sure that all the computers on the network are using a different ne~address.