Samsung
Activate a Downloaded Keyboard auf Samsung
Samsung phones ship with the built-in Samsung Keyboard active, but any keyboard you install afterward from the Play Store stays inactive by default. This is a safety setting in One UI: a newly installed keyboard app first needs to be switched on in the system before it can be selected anywhere, including in the keyboard switcher above the on-screen keyboard.
If you skip this step, the app appears in your app list, but it will never show up when you tap the keyboard-switch icon or try to set a new default input method. This often looks like a bug, but it is simply an inactive keyboard.
To turn it on, go to Tap on Settings ➔ Tap on General management ➔ Tap on Keyboard list and default ➔ Turn On a Keyboard ➔ Tap on OK. Confirming the on-screen warning is required, since Samsung asks you to acknowledge that a third-party keyboard can technically read what you type.
Once activated, the keyboard becomes available for selection, but it is not automatically set as your default. If typing still opens the old keyboard afterward, check the default keyboard setting separately in the same menu.