If you have a dedicated server or a vps, you may remove your clients from your firewall as needed from within WHM. Here's how:
1) Log into whm
2) Scroll all the way to the bottom on the left column
3) Click on "ConfigServer Security&Firewall" (under the plugins section)
4) Click on "Firewall Deny IPs"
5) Press the "control" + "f" buttons on your keyboard at the same time, and it will open a find/search window
6) Paste the ip address in the find window, and it should find it for you
***(if needed, the client can get their ip from http://whatismyip.com)
7) When you find the correct ip, simply highlight and delete it from your list
8) From there, scroll down and click to save the changes
9) You will be prompted to restart the firewall -- go ahead and click to restart, and then you should be set.