Wifi Connection Problems

I recently changed the wifi username and password on my office's modem/router (it is a modem and router in one). I changed it by typing the IP address into a web browser and going through those steps. It worked successfully the first day I did it and never had problems connecting to it again.

Once other office members tried to connect to the wifi with the new username and password it didn't work. They would be able to type in the password but then an exclamation point would pop up on the wifi icon in the upper right corner. What does that mean? How do I fix this problem?

Any advice will help!