Can you test the speed of wired connection from your laptop to LAN port of WR841N like you tested by NBN router? How about link speed of wireless network connection? Try channel width 40MHz and...

It means that V2 is new and has no newer firmware yet... You said "I am looking for the latest firmware for WA850RE v1.2 but can't seem to find it on the TP Link website. " Is it a mistake?

It depends... How about your router's Wi-Fi coverage? Any obstacles between the router and WN721N?

What's the operating system of your PC? Where did you see the rate? Can you provide a screenshot? When the adapter works, you are connecting to Wi-Fi using Windows built-in utility instead of TP-LINK...

http://LAN IP: port works locally, right? But if you test it remotely with http://WAN IP: port, it works indeed? Check IP settings on your local PC including default gateway and DNS server. If they...

Firmware of your W9980 is up to date? What's the version of it? Provide screenshots of status page when it is working and stops working. How often does it happen? Does it reconnect normally?