The dd-wrt web server guide changes the web admin port by killing the running httpd process and rerunning it with a different port. It has the unwanted side effect of being out-of-sync with the remote admin port setting. Thus, even if the admin port was changed to 81, remote admin will still connect to port 80 (the default). A cleaner way of changing the admin port is by setting the nvram variable
http_lanport into the desired port. This can be accomplished by running the following commands via telnet/ssh or web gui:
I tested this on DD-WRT v24-sp2 big r13401.