Does LTE Network and MIMO work on the move?

Question

mobile connection

I bought an antenna for a Huawei LTE modem. All directed. The modem has two antenna inputs. In the store, the seller said that since the task is to increase the quality of reception in the car when driving, it is better to take one antenna (into one input), since MIMO technology is for stationary objects. Well, the question here is simple - true or not true.
Operator Megafon, modem E3372
PS: I'm not interested in the incoming speed, but outgoing. 15Mbit and stability. Without antennas, there are sometimes dips in the urban area, even without cars. I'm trying to fix it.