1. You need configure a default router on 2811 router to point to the default gateway.
ip route 0.0.0.0 0.0.0.0 192.168.100.100
2. If you don have dhcp server in your internal network, you can configure 2811 router as a dhcp server as well. It will assign IP, gateway, dns to PC.
you can refer to the following link to configure DHCP server on IOS router.