Pepor's page

I would definitely allow the use of the Escape action to break free from Manacles.

You can take a look at the CRB pages 503/504 (Adjunsting Difficulty). I quote: "You might decide a DC should differ from the baseline, whether to account for PCs’ areas of expertise or to represent the rarity of spells or items. A DC adjustment represents an essential difference in the difficulty of a task and applies to anyone attempting a specific check for it."

And I also quote: "You might use different DCs for a task based on the particular skill or statistic used for the check."

For instance, you would raise the DC because they are using Athletics instead of Thievery, making it Very Hard (+5), and they are trying it without help.

You could also ask for a minimun proficieny in Athletics.

I think it does apply. According to the CRB, page 234:

When someone or something tests your skill, they
attempt a check against your skill DC, which is equal to 10
plus your skill modifier.

And:

Skill modifier = modifier of the skill’s key ability
score + proficiency bonus + other bonuses + penalties