One of NYGC's staff stations had no internet while the rest were fine.
Network icon at lower right screen shows a yellow triangular sign with exclamation mark. Thus, it's not a disconnection.
Solution:
Router's DHCP server has reached its maximum available IP. Somehow, it is unable to automatically recycle resources. Had to reboot router for it to release and renew all IPs.
I also increased Maximum DHCP users from 50 to 99.
Backed up router settings to both NYGC server and my laptop.
Note to self: Remember to realize this earlier in the future so I could do this remotely, save the time travelling.