It's odd that after firmware upgrade the web GUI display normally.
Actually it is caused by the browser cache. Each time the firmware upgrade, we would make some change for the web GUI. Some options would be deleted, and some would be added.
This problem affects most Yeastr products including which have the web interface: gateways, U Series, S Series, N Series and so on.
The solution is very simple: just clean your browser cache. After you learned this, you had started to know Yeastar products.