I couldn't sign up or even find the sign up button for any of the recommended instances or through the android app. I eventually found a sign up option by navigating to a post and trying to leave a comment which prompted me to sign up. I filled in the requested info and the sign up button didn't do anything or show an error.

I tried again using view desktop site (on Firefox Android) and then it worked but still anything but a smooth intuitive process for now it seems.