Can anyone tell me what the left and right connectors are for?
The reason there is two antenna connectors is because 4G LTE uses MIMO (multi in, multi out) to get better speed. So you should purchase another identical yagi (so long as what you got is good enough and not fake multi band or fake high dBi) and then mount them tilting in opposite directions, one at -45°, the other at +45°. You can try just using the one yagi you have, I dont think it will make a difference using the left or right input. I suggest you test mounting with the elements vertical rather than horizontal if you don't have the part needed to try it tilted, or make something temporary to try it, perhaps from tin.