But I'll guess you want to keep your hardware. If so, look up DDWRT and see if that's supported on your model. Here's a google about DDWRT and captive portals.
Before the link I'll write I don't give support for that. Just directions on how to find information. That is, I expect a mid-tier IT staffer to set this up.
Read from https://www.google.com/search?q=DD-WRT+CAPTIVE+PORTAL+SETUP&gl=US
My question is that I have a router and I want my employees to insert their information to gain access to WIFI connection but the router doesn't provide something like that, just to let you know I have the Asus RT-88U and there's no such option about portal or captive control...
What is the possible solutions I need some assist...