Thank you Nicolas!
I did some more investigating and it turns out the reason sudo apt update doesn’t work is because the Husarnet keys have expired (as someone noticed here). I updated them and was able to update apt, but the keys will expire in a few months.
Would it be possible for you send me the blank image file you used as a base for installing ROS on the Asus Tinkerboard? I would like to try installing ROS from a fresh Ubuntu image with nothing else on it. I have looked many places on the internet and have not had any success finding a .img file that will work with the ARM processor in the Tinkerboard besides what is provided on your website.