I have no idea on this. I tried searching online, but found no information too.

It's simple : there are no trains into Serbia except the Bar (Montenegro) - Belgrade train. This is due to engineering works and/or political reasons.

You'll have to take buses across the border (easiest solution) or take regional trains up to the border and walk across it (it takes at least a full day).

Haha I’ll give it a try.

Seems so bad for the economy to have the rail system broken. On the website (https://srbijavoz.rs) they don’t even have their train schedule, they say to call to get it.

Holy moly, Srbija voz fixed their schedule publishing and their train schedules show up now!