Cant login to Router

Did you try 192.168.2.1? Maybe there is IP conflict between your modem and WDR3600 when both of them using IP address 192.168.1.1.

I have a similar issue logging in and especially logging in remotely
I get a message saying administrator can only be logged in once

Logout admin account in LAN side.

I did that on site but still can not log in remotely

This login page style means that router implement session login. Only one user is allowed at one time. You need to log out first before log in again from another PC or browser. Sometimes we just close the browser incidentally, the router doesn't treat it as logout, so that it keep denying your secondary login.

Here are the things you can do when we see this message.
1. Check whether there are other logins are not closed, log them out via "log out" menu, don't just close the browser.
2. If you confirm the former login windows is already closed, open that windows again and log in, and then perform logout via "log out" menu. This helps for most of the cases.
3. If the above steps still don't work, reboot the router so that all sessions will be renew to 0. Or just wait for 10 minutes when all sessions will terminate automatically.

As you are changing the LAN IP to 192.168.1.1, after the LAN IP changed, the former session via 192.168.0.1 is not ended, in you case you need to reboot the router. No need to reset it to factory default.