This article confirms that Katelynn was last seen by her father on December 21, 2021 at her apartment when he dropped off McDonalds and hugged her. It states that footage from the Town Pump was obtained by police, but the contents of the footage have never been revealed.
Father of missing Sidney, Montana, woman hopeful for her return: 'I just want her home'
So now we can update her timeline as follows:
Katelynn was last seen on December 21, 2021 at her home by her father, who delivered a meal (and a vape, as per another interview).
She last used her phone on December 21, 2021.
Her father did not see her on December 23 when he dropped off a box on her porch. He did not enter the apartment at that time. He texted, but did not get a reply. He returned another day to find the package still on the porch.
In another interview on FB with WTTNRL, Katelynn's father said she last used her phone on December 23, 2021, but this contradicts the latest news article.
Whoever pays for her phone may have accessed the phone records, whether or not her phone was unlocked. Whoever she contacted may have gotten the date wrong.
This indicates to me that Katelynn disappeared on or soon after December 21, 2021, as that is the last time anyone saw her or heard from her.