When you configure the IKEv2 VPN on a WatchGuard Firebox, the users that connect will not register to DNS without the following changes on each endpoint:
Within Network Connections, right click the IKEv2 VPN WAN Miniport, and select Properties
Select the Networking tab
Select Internet Protocol Version 4 (TCP/IPv4)
If the Firebox is configured to hand out the internal DNS Server, no changes should be made here.
Select Advanced…, then select the DNS tab
Configure DNS suffix for this connection, and then select the check box beside Register this connection’s address in DNS.
At this point, when your IKEv2 users connect, they will register in DNS on your DNS Servers within your domain!