I am having all these problems lol. I think I’ll wait until Module 13… 
and even if I sorta manage it, the other strings are ringing all over the place or I just outright touch them XD It’s like I suddenly have no idea how to mute strings. I am going super slow and have absolutely no accuracy on the initial strike using “thumbs up” and no matter what I do, as soon as I start fretting I just get harmonics. For the first time I actually find myself being frustrated; this is going to be a very slow technique for me to learn 
Edit I should add, the action on my bass is low, too.
Edit 2 Well, after just a few minutes I was nailing the opening strain to Pearl Jam’s Jeremy slap-style about 40-50% of the time. So my spirits are back up lol. As far as the thumb ring, to my knowledge the ones that use one do so for the tone itself.