I have an idea.
Please save or print these directions, and be sure to know what the screens of the routers look like....
In your WRT54GS,
#1 Go to Setup -> Basic Setup.
That screen will look something like the screen that you see at
http://ui.linksys.com/files/WRT54GS/v5-v6/1.50.6/basic.htm
#2 Check what the Internet Connection Type is set to.
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If it it set the Automatic Configuration - DHCP, now in your WRT54GS go to Status -> Router.
Record the MAC Address there. You will need this latter on.
Click on release.
After you click on release, disconnect the WAN port from the B90-210015-04.
Connect the WAN port of the WRT610N to the B90-210015-04.
Login to the WRT610N with the user name and password of the WRT610N.
After logging in.
Go to .Setup -> MAC Address Clone.
The screen will look something like the screen that you see at
http://ui.linksys.com/files/WRT610N/1.00.00.17/WanMAC.htm
In there, click on enable.
Type in that MAC Address that you recorded, and click on save changes.
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If you find this is to hard for you after you press the release button in the WRT54GS:
#1 Turn off the modem and WRT54GS.
#2 Disconnect the WRT54GS from the modem.
#3 Wait at least two hours, before connecting the WRT610N to the modem.
Hint: In DHCP areas, there is MAC Address bounding that last at least two hours.
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If the WRTG54S is set to PPPoE, go to status -> router and be sure to press disconnect before disconnecting it from the modem.
