I found that the router setting in Time Capsule was already DHCP & NAT. I realized I had an Airport Express extending the network created by my Time Capsule. So I unplugged that. Then I saved the Time Capsule settings, where I had made no changes; that restarted Time Capsule. Then I was able to connect to the internet via Time Capsule and it stayed working for at least 5 minutes. So I plugged back in my Airport Express. Time Capsule was still connected to the internet and Airport Express enabled internet connections too. So maybe the problem was that I had forgotten to unplug my Airport Express that was extending the network created by Time Capsule. Thanks a lot for your advice.