Turns out that tethering doesn't seem to work on a Telstra Buzz with a Yomojo sim. DNS address doesn't resolve or something. I have tried tethering from my Note 3 with the Yomojo sim and there was no problem. Has anybody gotten it to work on a Telstra Buzz?
I'll add that the mobile data does work on the Buzz.
EDIT: found a fix thanks to Mountain-Man from wp!
I experimented with the APN, and now have the 'APN type' set as "default,dun" – no quote marks of course.