Thank you for your response.
The management appliance is running R81.10 and the Gateway (Cluster) is also running R81.10 with Jumbo Hotfix Take 95.
All I have done is: -
Add a new Trusted CA under servers along with a Subordinate CA.
Open the Gateway Cluster Properties, IPSec VPN, Generate a new certificate using the new Certificate Authority.
Under "VPN Clients" I have changed "The gateway authenticates with this certificate" to the new one. I have also done the same under "Remote Access" Support L2TP Use this certificate.
EndPoint Connect version E86.10.