I am looking for something similar to : Gate – Jietai Kare no Chi nite, Kaku Tatakeri. Basically, I want a novel were modern military fight against people coming from another world. Thanks. Edit : It can be a Royal Road novel too.
Impregnable Dreadnought There's honestly not that many novels which has Earth in a fight with another world, but there are way more novels which have 'modern' weapons in a fantasy world, and how the weapons got there is another matter.
http://www.novelupdates.com/series/seoul-stations-necromancer/ http://www.novelupdates.com/series/evolution-theory-of-the-hunter/ (I think? there is sometime since I last read it. but I think there is military and modern warfire.) http://www.novelupdates.com/series/god-and-devil-world/ http://www.novelupdates.com/series/galactic-dark-net/
If demonic hierarchy of standard western hell counts as fantasy... Additionally if it doesn't strictly require to be an Earth civilization (and you're fine with a bit digging) try looking for Hell's Gate by David Weber.