You need to boost the roots development. Those cups are good for the first week. Transplant into intermediary pots (1.5-2L pots), bury the seedling just right under the cotyledon leaves it will grow roots from the buried stem.
In soil, you’re not supposed to use any nutrients during 3 weeks after transplanting, any horticultural soil is pre-fertilized.
At this stage, you use only roots enhancers.