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