Did anything happen when you upgrading the router?

You should make sure your T2600 connect to the Internet. http://www.tp-link.com/en/faq-940.html

Actually, PADI timeout means the packet PADI cannot send out, which means your connection may have some problem. You can first check you cable connection.

If you can, maybe a data packet capture will help you to see what happened. Or you can reset the devices and have a try again. Then if it doesn't work, just contact the support of linksys or TP-Link.

Could you give more information about the problem?Have you ever check your cable and connection? And you can simplify your topology then have a try again.

Did you configure LAN to LAN IPsec VPN?