|Usually in cases like this, it's because your MAC address is registered with the provider that a replacement modem doesn't work. Normally I would advise you to "spoof" the MAC from your old router on the new one and it should work. However, you say you tried the old router again and it didn't so I'm a little stumped.|
Just to be on the safe side, plug the old router in again and power cycle the modem. When both are back up and running check. If you now have internet on the old router, then it's a MAC registration issue. All you need to do then is look through your old router's settings to find it's "external MAC address" . Write that down on a piece of paper and then connect to the new routers management interface, find where to change the external MAC on it (spoof) and put the MAC of the old router in there and it too should now work.
If the old router does not work at all, or if it does and your new one doesn't have a place to spoof it's external MAC, you're going to need to call your provider and get their help with this situation.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.