I need suggestions that help me finish this challenge.
Planks are one of those exercises that don't train anything directly, and you can't really train for it.
It is an awesome tool to measure overall fitness and progress, thus you train everything else and your plank times increase too.
Which means I would advise to train everything else... and eventually come back and recheck your plank times.
What to train: Shoulder muscles, back muscles, triceps, biceps, glutes. And of course:
core muscles.
You mentioned that your "ab" region started to get sore, the abs are really weak and slim muscles, you can't really train it in isolation, but you can train your overall "core" area (lower back, side etc.) which should help.
Here are a few examples.


Here are a few advanced exercises:
The modified candlestick might be one of the best. (Darebee calls them reverse crunches on picture 2)
I personally really like the Slant Board Situps (with something holding my ankles). (Darebee call them "sit-ups" on picture 2).
Just my two cents.
I might be wrong, I am not a professional.
I hope that helps.