Cannot hook up coaxial cable sent with upgraded equipment.

I was sent an email from Xfinity offering an upgraded router because our current router was outdated and couldn't support the speed we are supposedly getting through their service. I accepted the free upgrade and was sent the new router. Everything was fine and well with the product, but the coaxial cable provided to hook it up is too short, and we have no internal coaxial plugin outlet in that room. This is where when we first got our service set up years prior, they drilled a hole through the wall and routed the coaxial cable outside to a gray plastic xfinity box which contains a small mess of coaxial cables connected inside. I have no way of hooking it up myself and secondly the cable they provided is too short. Would using the old coaxial cable work by any chance or would that restrict bandwidth and speed?

They also asked for the old equipment back and did not provide a box or return sticker and I do not have my box for the old router anymore. I do not have a printer to print the label out for return, what should I do about these few things? Thanks to anyone who can help.