The nominal mAh of the 55Wh battery is 3572mAh
HOWEVER that is at 15.4V, while the Powerbank you linked is 20000mAh at 3.7V
So I’d actually recommend doing this the other way around, calculating the Wh of the Powerbank, because Wh is an actual representation of Energy stored inside the Battery, while mAh is only telling you half the story.
And multiplying 20Ah with 3.7V, we get 74 Wh, so that Powerbank is gonna charge the FW 13 about 1.3 times.
Given average battery life reports between 8-12 hours, that should give you between 18 and 27 hours of non-stop use (Laptop Battery + Powerbank),
I’d stick with the 18h estimate, both just to be sure and to account for Windows (assuming you’re using it) thinking “Ohh look I’m plugged in so I can use even more power now to run random background tasks”, but 18h of run time should still suffice for most if not all situations.
I don’t know about the fairphone specifically, but a decent amount of phones have pocket and pick up detection. They can detect when you take them out of your pocket or pick it up from like a desk and automatically turn the screen on, without unlocking the phone. There may also be a “double tap to wake” feature, which turns on the screen when you double tap it.
Both of these cause an at least noticeable decrease in battery life, but could be worth it if this is a big QoL thing for you.
If neither of those exist or you need the battery life, try only pressing the power button with your fingernail or sth, that’s what I did for some time and it worked just as well.