Small guide:
1. Open Google Play Store
Click on 3 stripes in top left corner
Go to settings
Click on autoupdate apps and don't autoupdate apps
2. Search Apkpure in google
Scroll down to more...
Scroll down to previous versions of Underworld Empire
Download version of your choice
3. Uninstall Underworld Empire
4. Open explorer app
Go to apps
Click on the file you just downloaded and install
​​​​​old version of UE (version 5.51)
You will get WARNING message it is not safe to install from outside sources.
Click install anyway.
Sometimes you have to go to Android settings and security and enable download from outside sources.
I can't help you with that because every new version of Android is different.

You still get Android updates but you have to update manually.

P.S. Do this at your own risk I don't take responsibiliy if something goes wrong!
It worked for me on Doogee Y9 phone with Android 9