How to Share Laptop internet to router via ethernet port

You can do it following this answer: How to share PC Internet over Wireless Router on Windows?

However there is another way. You can create a hotspot with your laptop. Here is a guide to Windows 7 and 8 http://www.howtogeek.com/214080/how-to-turn-your-windows-pc-into-a-wi-fi-hotspot/

A video tutorial for Windows 7/8/10 https://www.youtube.com/watch?v=Nh6gbrnYzS0

And a guide to Ubuntu http://ubuntuhandbook.org/index.php/2014/09/3-ways-create-wifi-hotspot-ubuntu/

With Win 7(pretty sure it's the same in Win 8), when you have the wifi signal and the Ethernet plugged in to the router, go to Network and Sharing Center, on the left there is a Change Adapter Settings, the select the wifi adapter and the Ethernet adapter together with Ctrl. Right Click one of them and there should be a Bridge Connections option, this will send your wifi to the cable.