Can't reinstall Calendar Storage not via phone, adb nor Reflashing NOTE: I tried this, but links are broken and things don't work.. I tried googling those tough.

Phone: Umidigi Crystal (2GB RAM version)

I got an option under the developer options, to root. I checked it. Since I use 
Google Calendar, I would get double notification from the android calendar, I accidentally tought Calendar storage was part of Android's too. Turns out Google's needs it. Here are my attempts to restore it:

Trying to restore from unistaller: a)Corrupted b)There already is that package (have searched all / for those names, no result) c)Try again later

Factory Resetting the phone: It didn't come back.

Trying to install a downloaded version of it (via adb and from phone, locally): Something conflicting with signatures I guess: 

C:\Users\Laptop&gt;adb install C:\Users\Laptop\Downloads\com.android.providers.calendar.apk
5135 KB/s (441556 bytes in 0.083s)
Failure [INSTALL_FAILED_SHARED_USER_INCOMPATIBLE: Package couldn't be installed in /data/app/com.android.providers.calendar-1: Package com.android.providers.calendar has no signatures that match those in shared user android.uid.calendar; ignoring!]


Trying to reflash: I have did, what I am told from multiple guides, but somehow SP Flash Tool just doesn't do anything.

Feel free to ping me for additional information, I really miss my calendar.
I am all good with backups, so I can freely factory reset it.
Right now I think, that easiest would be to get the flashing thing working.
Well, since this app is a system app, pre-installed by the vendor, there is no way to get it back other than by flashing the ROM file. You can't  install it through ADB since you are not rooted and trying to sideload would end up with errors because your recovery would not permit it.

Now only the flashing option left. Since it is a Mediatek device, flashing should be easy. All you need is SP Flashtool and a compatible ROM file with scatter, this maps your memory and tells the flashing software where each files should go, etc... (generally found at the manufacturer support website). 

Also, download Mediatek VCOM drivers this is important to make your decision be detected by the computer.(On Windows 8 and earlier, you need to disable the drivers signature verification otherwise the drivers won't be installed). 

Launch SP Flashtool, load the scatter file ( inside the ROM file archive). Now on SP Flashtool, click on Download, connect the phone while switched off. The flashing process should start (Read bar, Purple bar and then Yellow bar which can take some time to finish). 
Done (Big green check mark).

Power on your device. Note that the first boot takes several minutes to complete as the system is putting everything in places, just be patient and wait.  Android should now begin installing default apps. And your don. All back to normal.