That depends on the Roku device completely. If the Roku device has a built in wireless adapter it will be possible, otherwise you will need a cable. To find that out either check out the Roku user manual or look through the Roku network settings, for a wireless option. If you don't find either of the two then you're probably out of luck.
The connection between the Roku box and the TV is HDMI cable. Most Roku box have wireless but this is the connection between the box and the internet.
You problem is going to be you need to be close enough to the TV to attach the Roku via a HDMI cable but also close enough to the router to pick up the wireless signal.
They do make HDMI extenders that will transmit the HDMI signal over a wireless but they are very expensive and you have the same issue. If normal internet wireless is too far from the TV it is likely these special HDMI boxes will also be too far and these boxes are trying to transmit much more bandwidth than the normal internet ones.