Apk Mirror's APKM is an encrypted bundled app format which can only be installed using their own installer. UnApkm is a small utility software that converts an APKM to APKS which can be installed via an split apk installer that supports APKS format.
Instructions: The app doesn't have any GUI. Goto your favourite file manager, select an apkm file and open it with UnApkm. You will be asked to provide a save location. When the conversion is complete, you'll see a toast message at the bottom saying "success". You can find the apks file in your provided location.