Personally I have used Mobilism for years and it is simply the best site to download apk with pay options unlocked, although sometimes there may be crashes, from my experience can I say it, but it is not the most normal.
Have you checked that the apk you are downloading is compatible with your device? For example, depending on the age of your device and the version of Android you should download apk that are armeabi-v7a (old architecture) or arm64-v8a (newer architecture), although there are also apks that bring all versions in one.