I have a router from TPG that can operate on 2.4Ghz and 5GHz bands but none of my WIFI usb devices plugged into computers can use the 5GHz band being restricted to 2.4GHz. Is there any actual advantage in buying a usb WIFI device that can use the 5GHz band in terms of speed and reliability? Or should I stick with the 2.4GHz devices on my computers?
Wi-Fi nowadays work on either the 2.4GHz or 5GHz spectra.
Each spectrum has its advantages and disadvantages which depend heavily on environmental factors.
So, let's say your TPG router has both 802.11n and 802.11ac Wi-Fi, then connecting your computers using a dual-band (2.4GHz and 5GHz) AC Wi-Fi adapter to the 5GHz signal can give you better speed.
In terms of connection reliability, if your computer and router have line of sight (best case scenario) or are near to each other, your connection is more stable due to no interference from other 2.4GHz devices. Also, due to 5GHz's range limitation, your neighbor's 5GHz Wi-Fi may not even reach your home to cause any interference whatsoever. So if you live in a city or in a multi-dwelling building/apartment, 5GHz may pretty well work in your favour.
However, if your TPG router is a couple of rooms away from your computers, 2.4GHZ Wi-Fi may be more reliable than 5GHz. So if you live in a big house with lots of rooms away from the city where neighbors are far, you may want to stick with 2.4GHz.