This is the recommended guide for you to follow: https://documentation.meraki.com/MX/Client_VPN/Guided_Client_VPN_Troubleshooting#Windows_Error_789 If your MX is behind a NAT-T you might want to look into this https://docs.microsoft.com/en-us/troubleshoot/windows-server/networking/configure-l2tp-ipsec-server-behind-nat-t-device It is quite advanced and should come with all the red warnings about how you can make your operating system completely unusable and lose personal data, try at your own risk, etc.
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