Sorry, the probem is technical, not possible to reply without tech.
First of all: have you called your ISP (Internet Service Provider you get the DSL service from)?
Many DSL providers provision the DSL modem (activate it) for one device only. The fact that your Mac works indicates that it is working and provisioned for your Mac's unique ID. Just plugging in the router will not work in this case. Call the DSL provider and find out if this is the case.
You then have a choice - you can often get the router to "clone" the MAC address from your Mac, so it 'spoofs' the DSL modem into thinking it's still talking to Mr. Mac (consult the router's instruction manual). Or you can ask the ISP to provision the DSL for the router's unique MAC address.
If that isn't the problem, then it is likely the way that you have the iMac set up with internet connection information. When you add the router, here's what happens. The router talks to the modem, and the modem issues the router an IP address on the network, and a DNS server IP address for the "phone book" of the internet (Domain Name System)
Then your Mac talks to the router, and the router passes along the DNS address, and creates a special internal IP address for the Mac (and any other device connected in your household network.)
In order to do this, your Mac has to know the address of the router, the "sub-net" that it is in, and to go and ask for the network numbers.
The Mac's Network Preference Pane: Built-in Ethernet connection: TCP-IP properties should probably be : Use DHCP
If all goes well, it will get all the info it needs from the router.
If not, you may have to manually fill in the IP address for the Mac, the Gateway/Router address, and the DNS address. Consult the manual that came with the router, there are too many variables to cover here. The manual will also show you how to use your web browser to check the satus of the router.
If you have already tried the instructions for accessing the Router settings with your Web browser, and cannot reach the router's internal web settings page, then I would suspect connections and cables. Try to simplfy the setup as much as you can, and substitute some known-good cables.
First of all: have you called your ISP (Internet Service Provider you get the DSL service from)?
Many DSL providers provision the DSL modem (activate it) for one device only. The fact that your Mac works indicates that it is working and provisioned for your Mac's unique ID. Just plugging in the router will not work in this case. Call the DSL provider and find out if this is the case.
You then have a choice - you can often get the router to "clone" the MAC address from your Mac, so it 'spoofs' the DSL modem into thinking it's still talking to Mr. Mac (consult the router's instruction manual). Or you can ask the ISP to provision the DSL for the router's unique MAC address.
If that isn't the problem, then it is likely the way that you have the iMac set up with internet connection information. When you add the router, here's what happens. The router talks to the modem, and the modem issues the router an IP address on the network, and a DNS server IP address for the "phone book" of the internet (Domain Name System)
Then your Mac talks to the router, and the router passes along the DNS address, and creates a special internal IP address for the Mac (and any other device connected in your household network.)
In order to do this, your Mac has to know the address of the router, the "sub-net" that it is in, and to go and ask for the network numbers.
The Mac's Network Preference Pane: Built-in Ethernet connection: TCP-IP properties should probably be : Use DHCP
If all goes well, it will get all the info it needs from the router.
If not, you may have to manually fill in the IP address for the Mac, the Gateway/Router address, and the DNS address. Consult the manual that came with the router, there are too many variables to cover here. The manual will also show you how to use your web browser to check the satus of the router.
If you have already tried the instructions for accessing the Router settings with your Web browser, and cannot reach the router's internal web settings page, then I would suspect connections and cables. Try to simplfy the setup as much as you can, and substitute some known-good cables.
