I have a RT-AC68P router. I can successfully login, but any time I try to change anything it keeps bringing me back to the login page.
This implies im not logged in, but I can still navigate the router web pages and see existing sensitive information including passwords, which implies I am logged in.

Stuck, and unable to change anything in the router or even apply firmware upgrades.

Not really sure what to tell you here. That's a router; the only real thing to try would be holding the reset button down and going back to factory defaults, providing you've tried the VERY obvious step of closing/re-opening your browser. And disabling any pop-up blockers/ad blockers/noscript tools you have running, which all may interfere with a firmware-driven, single-purpose webpage.

I guess you powered it down already. That should remove any old log in locks.

The choices for the problem I see are some issue on the browser you are using, some connection problem, some hardware/software issue on router.

I'd try a phone or tablet first to see if results are equal.

I'd connect directly to a computer and nothing else and see.

I'd hit that reset button and try again.

I'd go out and buy another if all else fails.

Sure do like the Asus cause they support 3/4G usb modems. Means I can put almost any phone on it and use it over lan.