I recommend looking into SideStore instead. It’s similar to Alta store but you once you installed the SideStore on your iOS device you can manage the other apps without a computer.
It requires a computer to install the Store app. Then you can renew on your phone and install apps without a computer. It also requires a computer is you go let your alt store app expire.
Yep - that's because Apple has not released a stable way to create alternative app stores. Developpers still have to find roundabout ways to offer an alternative store. I am sure they will adapt as soon as they can get a stable stand-alone app working in the apple ecosystem