They probably have a popup-blocker running.
I don't think that's it as the CP doesn't use a pop-up AFAIK, plus I use a popup stopper and it works for me. Thanks though.
Did you add your internal/ISP DNS servers IPs to the captive portal "Allowed IP addresses" list?
It happens when a client request an internet page and can't DNS resolve it, the CP loginpage won't show up.
I did not add them, but i'm going to right now, not sure when the next time I can test the theory out is as i'm not sure when Canada Revenue will be back but i'll backup and remove all the Pass-through MACs and i'll find out eventually if it works and let you know. Thanks for the tip, hopefully it solves the problem.