It sounds like you've done a good job troubleshooting the network and NTP side of things.
Disable Automatic Time Zone:
Go to Settings > Time & Language > Date & time.
Ensure Set time zone automatically is turned off.
Manually set the correct time zone to W. Europe Standard Time.
Go to Settings > Privacy > Location.
Disable Location services to prevent Windows from changing the region based on the location.
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In Start Search type Gpedit.msc and hit the Enter key:
Navigate to Computer Configuration > Administrative Templates > System > Time Zone.
Enable Set the time zone and set it to W. Europe Standard Time.
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Look for any events related to time changes in the System logs under Event Viewer.
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Reconfigure NTP:
Open Command Prompt as Admin and type the following commands. Replace NTP_server1 and NTP_server2 with your actual NTP server addresses.
w32tm /config /manualpeerlist:"NTP_server1 NTP_server2" /syncfromflags:manual /reliable:YES /update
net stop w32time
net start w32time
w32tm /resync