1. It should be as simple as typing router's IP address in the browser.
2. That is odd.
Have you tried to reboot or reset your device to factory settings?
What device do you have and are you using the latest firmware, could you update, if not?
If the above do not help, I would like you to attach a troubleshoot file to your question. Please, replicate the issue by connecting a couple devices via WiFi, some logs should be generated in relation to DHCP, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into the issue.