Unfortunately this feature only works per browser. It caches where you are and can take you there when you go to the thread. A small pop-up appears in the bottom right hand corner to take you to the last unread post, but if you read on your phone and then open it up at home, it doesnt pass the information along. Some forums do this by building in a database system, but it does slow down the site a bit as it has to check against the database every time you open a thread.
This must be where I was getting confused then, because I often read on my phone then on my computer then on my phone again. The old forums synced across both irrc, so I never had this problem before, leading me to think it was broken here.