You may need to secure a Visa for entry into Montenegro. Please try to arrange to get the Visa on time as it might take several weeks to process the paperwork.
To find out whether you need a Visa for entering Montenegro please consult the web page of Ministry of Foreign Affairs of Montenegro.
If you decide to travel via Croatia, you might need a visa for Croatia, too. To find out more about this, please check web page of Ministry of Foreign Affairs of Republic of Croatia.