Voip Clients on Android version 5.X.X and higher cannot connect to FreePbx's VPN IP Address from OpenVpn 1.1.17

Hi,
I bought commercial module System Admin with VPN server support to connect mobile SIP Clients on multiple Android mobiles phones. It works well with old Galaxy S4 (Android version 4.2.2) but does not work with Galaxy S3 (2016, Android version 5.1.1) and Xiaomi Redmi 3S Pro (2016, Android 6.0.1). All tested phones have the same version of OpenVPN Connect 1.1.17(build 76) and Zoiper 1.40 for Android. In fact, OpenVpn clients of all mobiles phones connect to FreePbx's VPN server without any problem, but the phones with Android 5.X.X, 6.X.X cannot connect the FreePBX IP address (default is 10.8.0.1). There is no ping at all too between phone and FreePBX. All tested phones were connected to the same service provider. Profile configure file for OpenVpn was the same for all phones.
Thank you in advance.