How can I share my PC's internet with my raspberry pi 2B?

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networking

wireless

routers

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Ideally I could just use a flylead between the Ethernet ports on each device, but even then I think some - for me - heavyweight configuration would be necessary. Avoiding wiring between rooms is a very high priority, and my main ADSL WiFi router is in the lounge, and I want my Pi in my bedroom.

Next up, I have a TP-LINK 3G/4G Wireless Router, which has a USB port for a 3G/4G modem, but also a WAN port for backup if the USB modem fails. I'm thinking my best bet here is connect my PC Ethernet to the WAN port on the router, but if I just do that straight, the router says there is no WAN present. I imagine, if possible, it would take some networking setup on my PC and on the router, to get them onto the same subnet or something, or to make my PC look like a WAN.

Is any of the above possible, and how should I go about it? I also have a Wireless Range Extender, with one Ethernet port, but I imagine that is for setup, or connection to the 'LAN' (my PC) for an internet consumer, not to my Pi as an internet provider.

What I have tried:

I have tried connecting my PC (Lenovo T520i laptop) to the WAN port on the Wifi router, with no joy. If I connect it to a normal LAN port, I can at least reach the router's control panel, but with the WAN port, the router says, "No wan!" and I lose all comms.

I havne't tried anything more because I don't want to fiddle with the network settings on my laptop and perhaps lose all comms here as well, the implication being I lose Google and forums as well;