If that isn’t strong enough then you will have to bring out the big guns. Steel wool and soap. Normally you are not supposed to use either of these things on an iron skillet because it will remove the seasoning.
However, if the pan wasn’t seasoned properly to begin with you will have to redo that any ways so you must get all the crap off the pan by any means necessary.
Cast iron is tough! It’s almost impossible to completely ruin an iron skillet. If you think your pan is beyond the point of return, have hope- you can bring it back to life!
For regular maintenance any type of oil or fat will do but for your first seasoning you should use oil that has a smoke point higher than 350 degrees. Try lard, Crisco, or avocado oil.