As far as I'm aware, you cannot make a payment toward your device balance online, nor can you pay it off online. There may exist a way to do it that I've never seen, in which case someone else might say. You can call a Telus store and ask. Don't call Telus support, unless you have time to sit on hold into 2019.
You cannot do this online. You will need to go into a Telus store. Not sure why you cant do it online and I agree it would be a good idea. Usually most people only want to pay off their device balance to upgrade their phone so probably not much demand for that option.