Reason: The phone and the PC only have one IP, so they cannot use net at the same time. The ports of RJ45 phone are similar to a 2Port HUB. It does not have the router function of NAT, so it cannot serve as a router.
You just need to add Router or proxy server with NAT function in the front of your phone. Using the LAN port of your phone to connect to the Router or proxy server and using PC port of your phone to connect to the PC. In this situation, your phone and your IP can acquire different IP, thus get online at the same time. If the problem is not solved, contact your agent to check any problem of the network connecting ports.