I can not say how many times I have started reading a book that is supposed to take place a long time ago and yet they have modern inventions. It's very frustrating.

Before you start writing a fanfiction, you need to research the time period the fandom you're writing for takes place in if it isn't now. Black Butler for example, takes place in the 1800s, and yet, I've read stories where the characters dress modern, have phones, and watch movies. What?! A fanfiction in that time period should have the character in time appropriate clothing, and should have them speaking properly, and they should not have technology. An exception would be if the character was teleported with their electronics there, but even then internet didn't exist so they wouldn't be able to use them.

Really, just do your research. A little googling about the fandom and the time period it revolves around can go a long way to improve your story and stop confusion about weird technology, clothing, or mannerisms you might write