As I read https://www.enpass.io/, you will need a WebDav server to store your passwords. At least in my opinion, doing this on your router is not a wise idea from a security standpoint.
It may be easier to find tutorials for setting up, for example, nginx with webdav modules (which do not require a special file system) on something like a Raspberry Pi running Raspbian. You can then port-forward to that device.
To add on to the info provided by Jeff and Ilichi, openwrt does not have any other users/logins by default, so you’ll only need to worry about system wide configs, which are usually in /etc or a sub directory or two below it.
I do hope this info is helpful. Again, if you are still stuck I’ll help as best I can.