I purchased a airport express, but it doesn't work very well

If you are saying that you have tested the long Ethernet cable with your Mac to make sure that it is working, now it is time to configure the AirPort Express.

Disconnect the Ethernet cable from the Mac

Connect the Ethernet cable to the WAN "O" port on the AirPort Express

Power up the AirPort Express for a few minutes

Hold in the reset button on the back of the Express for 7-8 seconds and then release the button

Allow a full minute for the AirPort Express to restart

Make sure that the WiFi is turned on at the Mac

Click the WiFI menu at the top of the screen

Click on AirPort Express

AirPort Utility will take a few seconds to analyze the network, and then display a screen that looks like the example below, except that you will see a picture of your AirPort Express.

Wireless Name......Type in the name of the wireless network that you want to use

Base Station Name.....Type in a device name for the AirPort Express

Password.....Type in a password for the wireless network and device

Verify.....Type in the same password to confirm

Click Next

The utility will set up the AirPort Express with the settings that you have specified. When you see the message of Setup Complete, click Done

Now you can log onto the wireless network the AirPort Express is providing

Does the AirPort Express connect back to your main router using a wired Ethernet cable connection?

or

Does the AirPort Express connect back to your main router using a wireless connection?

What is the make and model number of your "main" router?

Do you have a Mac laptop that can be used to check some settings?

I think it is a wired Ethernet cable connection

It is either a wired or wireless connection. Can you confirm on that please?

If it is wired, did you test the Ethernet cable to make sure that it was working.....before....you installed the AirPort Express?

The dotted line on the screenshot shows that the AirPort Express is connecting via wireless.

If the AirPort Express is connecting using a wired Ethernet cable, then the line will be solid, not a dotted line.

Please test the Ethernet cable connection by turning off the WIFi on your Mac and then connecting your Mac to the Ethernet connection, to make sure that you can get a good Internet connection.

Sorry, but not with your Rogers router. The AirPort can only wirelessly extend a signal from another Apple AirPort router.

So, the AirPort Express must connect back to your Rogers router using an Ethernet cable connecton if you want it to provide additional wireless coverage on your network.

Post back when you have the Ethernet cable connection working, and we'll tell you how to set up the AirPort Express to do what you want.

What device will you use to set up and configure the AirPort Express?

If the long Ethernet cable is not working correctly, the AirPort Express will never work correctly.

I am not convinced that the long Ethernet cable is working correctly.

You do not use the "extend a wireless network" setting when the AirPort Express is using an Ethernet connection. The "extend" setting is used only if the AirPort Express will connect to another Apple AirPort router using a wireless connection....not Ethernet.

Follow the instructions that I provided. If you want the AirPort Express to extend the signal from your main router, set it up to use the same wireless network name and password as your main router.

If you have a Mac laptop, an Ethernet to Thunderbolt adapter is a very good investment, so that you can test both wired and wireless connections.

Thunderbolt to Gigabit Ethernet Adapter - Apple

Sorry, but no. You will need to troubleshoot the Ethernet cable and Ethernet connectors....before....you can set up the AirPort Express again.