Hi Mendel,

without some buses in between it is at the moment not possible to reach Montenegro as international trains are mostly cancelled (e.g. Zagreb-Belgrade).

The train from Belgrade to Bar (Montenegro) is running and very scenic. You could take trains until the Serbian town of Subotica (next to Hungary). From there a bus to Belgrade and then continue by train.
For Serbia you will most likely need a PCR test on arrival (even if you are fully vaccinated) - so keep that in mind!
Other option is taking a train to Split in Croatia and then take bus(es) to Montenegro from there. A stopover in Dubrovnik will definitely also pay off if you choose this route. This route is also scenic (by bus) and follows mostly the coast ;-)

Have a good trip