I ended up uninstalling Ark for Win 10, which I had installed via the Microsoft Store, and then reinstalled it using the XBox App for Windows. I then subsequently only launch it via the XBox App. Since I've done that I haven't had any issues joining the password protected Nitrado server via Ark's built in server browser.
Nitrado now has an official Discord server to bring communities, friends and other gamers together!
Join the Nitrado Community Discord now and share your experiences and knowledge with others!