Regarding the install from FlexLink web pages, you need to do this (https://www.howtogeek.com/313433/how-to-sideload-apps-on-android/):

How Enable Sideloading in Android 8.0

1.	Open Settings > Apps & Notifications.
2.	Expand the Advanced menu.
3.	Choose Special App Access.
4.	Select “Install Unknown Apps”
5.	Grant the permission on the desired app.

This can be seen as disabling security, but also not, as our app is an official FlexLink digitally signed app.