R6120 unable to connect to wireless devices

Recently attempted to install the R6120 Wifi Router yet am unable to connect to the network from my pc or my Roku tv. When I plug modem cable into the router all lights turn green but when I attempt to log into the network from my pc I can find the name of the network in my list but am unable to receive internet without taking modem cord from the back of the new router and plugging it directly into pc. I do not have access to a smart phone and was not able to use the Nighthawk App to complete the install. Is there some way to be able to find and join the new router network?