Answer by Tom for Asus RT N16 router. Prevent redirection when there is no internet connection

1. Go to the Administration menu and open the System tab.
2. Enable the telnet server by setting “Enable Telnet” to “Yes”. Click Apply, wait for the changes to complete, then telnet to your router’s IP address and log in with the same username and password you use for the web interface.
3. At the command prompt, type: nvram set web_redirect=0
4. Changes should take effect immediately. This seems to prevent it, except for certain situations you probably won't run into, like when where the WAN connection is unconfigured.
5. Disable the telnet server again when done (if you don't need it).

(From here, which also includes further detail: https://moeffju.net/blog/disable-asus-rt-n66u-error-redirect. After reading the linked source code I'm even more inclined to run something like DD-WRT on my RT-AC68U!)