IT infrastructure is fairly developed.
You can buy pre-paid SIM card for a few euros. If you need more you can sign contract with some of the telecom providers. Currently we have three operators: https://www.mts.rs/ , https://www.telenor.rs/en/consumer/ and http://www.vipmobile.rs/home.8.html
When you rent a flat there are basically two options, ADSL or Cable internet connection. The speed goes up 150 Mb/s for some cable operators. I have personally used ASDL for years and it was really good.
WiFi in cafes is always free, you should just ask for password. Even if it is not written that they have Wifi, you should ask for password.
There are few co-working places around the city. Just google them. The price is around 150 EUR/month. For that price you usually get your own place, internet connection, and some refreshments.
If you want to rent a car, you need "International driving license".
ATMs are all around the city. The most popular bank cards are Visa and Master Card, but you can pay in shops with American Express or Dinners on some places.
All prices in the shops are in Serbian dinar (RSD). When you want to exchange money do that in Exchange offices. They have better prices than banks. Serbian law for exchanging foreign currency is pretty strict and you should expect the price which is advertised. Sometimes you can get even better price for foreign currency if you are exchanging larger amounts (500 EUR +).
Younger people usually speak English, but unfortunately on many places they don't. It is especially irritating when they don't speak English in restaurants.
Serbian restaurants are not good place for vegetarians. Although, there are few vegetarian restaurants, majority of them are only for people who eat meet.
Restaurants usually serve traditional Serbian food, which means lot's of meat in different shapes. There are Italian restaurants which are pretty popular, but there isn't too many Asian or Indian restaurants.
This is my small contribution to this topic.