score:4
You can buy T-Mobile Montenegro Prepaid card. They have good coverage in Zabljak area (coverage map) and they offer prepaid data packages:
Also when you buy a prepaid voucher, you get a data bonus (between 50 MB and 2 GB)
Another option is to buy a Telenor prepaid card. More than 99% of populated territory of Montenegro is covered with their network (coverage map). It seems that they offer only GPRS for prepaid cards (0,06 EUR / 100 KB)