Switch question - load and traveler and such

I'm replacing a dumb switch with a smart dimmer. It's just a single switch, but there are 3 sets of wires. One coming from below, which I assumed was the hot/line, and two from above. Neutrals were not linked into the old switch, they were just nutted together. I have to put the neutrals on the new switch though (I assume - maybe that's only for one set?). I got it installed, and it worked in the new location, but it also made it where I can't turn on the light in the adjacent area without the new switch being on. It was not (shouldn't be) a 3-way, the other room is a 3-way on its own and worked independent. I tried to move each of the 2 black wires that I had in the Load spot to the traveler spot on the switch, but that didn't work (either way - the light I in the room worked with the switch, the other room didn't work at all even with all switches on). It only "works" when I have both in the load spot, but again it creates this new situation where the new switch has to be on for the switches in the other room to work. Does this mean I have the load one wrong? Or did I jack up that other room by putting all those neutrals into the switch? I don't want to just keep swapping the black ones around (all are the same color), but I might end up doing that. I don't really want to guess differently on the line, I'm pretty sure that will be the one coming from below.