The speed limits in Serbia are 60 km/h (38 mph) in built up areas, 100 km/h (62 mph) outside built up areas, and 120 km/h (74 mph) on motorways.
In Serbia the motorways are numbered and prefixed with the letter A, the non motorway main roads are numbered, the smaller roads just have the names of the town that they lead to on the signs.
The Serbian motorways and the non motorway main roads that I rode on are of a good standard, the condition of the smaller roads are generally good but there is always the odd exception.
The motorways signs have a Green background with White numbers for the motorway number, the non motorway main road signs have a Blue background with White numbers, and signs for the smaller roads have a Yellow background with Black names.
Serbia are not in the EU so they still have their national borders and depending on what insurance policy you have for your bike you may be required to purchase insurance for it at the border. They also do not use the Euro in Serbia and petrol at the time of writing was the equivalent of about 1.40 Euros per litre.
I never saw any signs for speed cameras and I never saw any patrolling police vehicles.
My personal experience of biking in Serbia was that it had very few roads that I enjoyed riding in. Only once I approached the border with North Macedonia did the roads get more to my liking.
To buy a SIM card for a mobile phone all you need to is to go in to any small moblie phone shop that sells prepaid SIM cards. These shops are mostly mobile phone accessory retailers and they are everywhere. No ID is required and when I did this the cost was around 10 Euros for one month of internet.