Can't connect L2TP VPN

Hi everyone, i have a atp 500 firewall. There is an L2TP configuration made by xyxel distributor.

Genarally i have no problem but some clients can not connect with l2tp.

I 've tried deactive client's firewall, network protection and antivirus program but this is not helped.

I can access to my local using l2tp with same account and different device?

Is there any configuration i can fix this issue on client's side.

Thank you for your help,

Mahir.