TP-Link N600 TD-W8980 admin page:
Connect to the router with a physical cable or wifi.
Navigate to http://192.168.1.1 in your browser. Default login is admin/admin.
If you can't login:
Try another browser; e.g. the login form always fails in Brave 0.25.2
Hard reset the router: With router powered on, press and hold the small Reset button for ten seconds. This resets everything including e.g. internet config, wifi config, port forwards, admin page login.
 
After you're logged in:
Consider saving all settings, at System Tools > Backup & Restore.
Consider changing the router's login from the defaults, at System Tools > Manage Control.
If you already tried these instructions and they didn't work:
Consider starting from scratch at System Tools > Factory Defaults. (This is the same as the 10-second button press to hard reset.)
 

Connecting the to Bigpipe UFB:
Navigate to Operation Mode in the left panel. 
Select Wireless Router Mode and click Save.
Wait for the UI to indicate that system reboot progress has reached 100%. 
Refresh your browser page.
Navigate to Network > WAN Settings. 
If it still mentions DSL-specific configuration options like VPI/VCI:
You've got the wrong Operation Mode, didn't reboot the router, or didn't refresh the browser.
Set Connection Type to PPPoE.
Input any non-empty, non-whitespace username and password.
Click Save.
 
At this point LAN (hardwired) connections should work. 
 
Configuring WiFi, either/both Wireless 2.4GHz, Wireless 5GHz: 
Navigate to Basic Settings, configure, and save. 
SSID: name of your choice
Region: New Zealand
Mode: "11bgn mixed" mode by default, but consider "n only" or "bg mixed" if you experience device connectivity issues.
Channel: "Auto" by default, but consider using Wifi Scanner phone app to find an uncongested channel
Channel Width: Auto
Enable SSID Broadcast:
Checked for visible network
Unchecked for hidden network
Enable WDS:
Unchecked if this is your only router or you don't know what it is
Navigate to Wireless Security
Configure WPA/WPA2; Personal; PSK, and save. 
 
If you're setting up a guest network, especially one without a password, consider:
Navigate to Wireless Advanced
Check "Enable Client isolation" (may be named "Enable AP Isolation")
This denies routing between devices on this wifi network (so, router + internet only)
 
Repeat these steps if you're using both frequency bands. Your 2.4GHz settings are not applied to the 5GHz network and vice versa.
 
Navigate to Dual Band Selection
Choose 2.4GHz and 5GHz by default, but consider only 2.4GHz or 5GHz if you experience interference or device connectivity issues.
 
Turn the physical WiFi switch on the router to on (the green dot.)
 
Once you're done:
Consider saving all settings, at System Tools > Backup & Restore.
Consider saving admin page and/or wifi credentials in a password manager.