In my Network, user are downloading the VPN Clients using mobile data, then they are connecting to my network to access the internet, So my DNS ip is assign to his /her device, in DNS itself we have AD, ADS not able to auth

In my Network, user are downloading the VPN Clients using mobile data, then they are connecting to my network to access the internet, So my DNS ip is assign to his /her device, in DNS itself we have AD, in my network have given ADS user name and password to access the internet, the above case if VPN client is enable other they are  not able to authenticate. since two month same issues happen but sophos support team not able to resolve it.