what model do you have? If it's an all-in-one router like a HAP AC, it should be horizontal, best coverage would be if it were flat on a wood shelf or similar.
Aren't Mikrotik part numbers just the most amazing things? For something like that, the antennas are omnis, the RF pattern coming off each one is an oval. Mostly vertical and maybe with the ones at the sides a bit at an angle is a good choice.
Visualize if you were to take a pencil and skewer it lengthwise through an oval shaped fruit, the fattest part of the fruit would be the highest gain part of the antenna's RF pattern.