Unable to update Iphone 7 plus from ios 11.2.1
I've been trying to update my iphone, it is currently on ios 11.2.1 when I go to settings and click on "Software update" it says "Unable to Check for Update" even when I try different networks. I also tried connecting it to itunes and updating it there, I updated itunes to the latest version and downloaded the new ios update but after its done "extracting software" and "backing up iphone" it gives me the error "The iphone could not be updated. An unknown error occurred (4000)".