If the 802.11n or 802.11ac AEBS or TC is connected directly to the broadband Internet modem or combination modem/router or gateway device, verify that the device is in bridge mode, i.e., if the device provides NAT & DHCP services, disable them.iOS: Access will require a third-party app, like FileBrowser.
The user name can be anything you like the password should be the disk password for your base station.
Ref: How to transition from Back to My Mac - Apple Support.As of JApple's Back to My Mac service has been discontinued.Mac running OS X Lion 10.7.5 - macOS High Sierra 10.13.6.Both must have a publicly reachable IP address, a firewall that allows remote connections, and support either the UPnP or NAT-PMP IP protocols. An AEBS or TC running 7.6.1+ firmware or a third-party router/gateway device.Using Finder, verify that you can access the AirPort Disk from the local network. The methods presented are not necessarily in any particular order. There are at least four methods to access a shared AirPort Disk over the Internet from a remote Mac or iOS device. NOTE: For the purpose of clarification, an AirPort Disk can either be an internal HDD in a Time Capsule (TC) or an external SSD/HDD attached to the USB port of either a TC or an AirPort Extreme base station (AEBS).