Yeah, if the app or Os/ firmware updates is too big . You can only download thru wifi, no matter what kind of data plan you have. It has been so since wp7. Similar restrict to some android device too, but some others not.
Yeah, how come you can get wifi access? Again which coutry op are from? Another work around which i did once when i was in cottage is put your sim or you have another sim in another phone and create a mobile hotspot and download
Settings, Network and Wireless, Data usage
Use cellular data if there is no Internet access over Wi-Fi: ALLOW cellular and SIM
Data connection ON Phone update If asked to allow updates over cellular answer yes Data usageclick edit and then set data plan to unlimited and then restrict background data to Never