Range Extender Has Lost Its Range

[Cloverjo]

Happy New Year to you. Can anyone help, please? I have desktop downstairs (Windows 7) connected by ethernet cable to a broadband Wifi router (3Com Office Connect 3CRWE754G72). Upstairs I have a range extender (TP-Link TL-WA730RE) so we can use our tablets and laptops.
The range extender has lost its range. All the necessary lights are on, except the one which says its receiving the Wifi signal from the router. When I press the connecting button, the light flashes to say it's searching for the signal and then goes out. I have tried turning each device off and on again. I have downloaded the instructions for the extender
http://www.tp-link.com/en/article/?faqid=14
but am getting confused about what needs to be connected to what. If I connect the two devices by ethernet, something complains about not being on the internet. I'm sure it wasn't that complicated when I first set the system up.
Can anyone explain simply what I need to do to get the range extender working again, please?

[Gromit]

I think you may have a blocked port on your router.
You will need to reset it through a browser on your computer.

This explains
http://www.pcworld.com/article/244314/how_to_forward_ports_on_your_router.html

[Cloverjo]

Thanks for your reply, gromit. I was doing ok until I got to the point where I have to find a tab to do with ports on the router's control panel. I can't find anything to do with ports in there.

[Gromit]

It should be in the router/modem instruction book. If you no longer have it, it is probably available as a pdf if you search for your model.

Alternatively, your ISP help should be able to talk you through it if you can listen Indian.