Promos can never help you, they can only hurt you.

From my observations of the promo system, you will be demoted if you lose even a single game at 0 LP, even if you have over 50% win rate, if that loss is not within the first 5 games.

Since this is true, it means that a player can actually win an arbitrary amount of games, and still be stuck. Example:

You are division 3, 75 LP in plat or diamond, so no promo helper.

You win 2 to get to promos with the standard +20 LP gains, 2 to get division 2, now you are division 2 0 LP after +4 wins.

You then win 3 games in a row to get 60 LP

Afterwards, you lose 4 games in a row. You have played 7 games total, outside the protected games, and thus demote back to 75 LP.

You have won net 3 games. Yet your LP has not moved at all. And the thing is, you can repeat this again, and again, and again.