Your MMR should be ignored when you get to your promos
If someone is at a much higher MMR than their division, the game tries to make them go 50-50 in their promos by matching them against people with the same, much higher MMR than the division they're trying to get into (or by giving them much worse teammates). It shouldn't. You should just play your promos against people in the division you're in or trying to reach.
For example, when you reach promos for gold 5, you would be playing exclusively against players with gold 5 MMR. Once you win, you go back to playing with/against people with a similar MMR to yourself. That's how you advance quickly, but promos are an unnecessary wall that forms due to being on a win streak and then having to win 3/4, or 4/6 games in order to advance when the system is designed to make you go 2-2 or 3-3.