When you're looking at the unread posts by using the unread content button, it's not possible to do it because of some sort of glitch/coding error by the providers who built the forum (there are workarounds I was looking at but I don't fully understand them, I need to do a bit more digging). It's also not possible to look at the 'last unread post' for the same reason, which is a major pain.
However if you are browsing the forums in general, you can go to the last page by clicking on whatever number is last (attached), but still not the last unread post