How do I wake up for Fajr?

Phones

Turn the Fajr adhan on, add the Fullscreen Alert, and stop the battery saver muting it.

What wakes you is the Fajr adhan itself, and three settings decide whether it actually sounds.

Settings › Notifications › tap Fajr. Turn on Enable Notifications, then Sound, then pick an Adhan Sound.
Turn on Fullscreen Alert. On Android it lights the screen and shows the alert over the lock screen instead of leaving a notification you sleep through.
On Android, turn on Override Silent Mode on the Notifications screen — without it a silenced phone stays silent at Fajr.

In the appOpen Notifications

If it still does not wake you

Run System Health Check. It tests every cause, sets up again any adhan this device has lost, and puts a FIX button beside whatever needs you.
Battery savers are the usual one — Xiaomi, Samsung, Huawei, Oppo, vivo and OnePlus each add a switch on top of Android's.
Add a Pre-Adhan Reminder on Fajr for a nudge a few minutes before, so you are already awake when the adhan comes.

In the appRun System Health Check