At the computer on the client side of the SoftEther connection, in the Server Manager, do this:
1. Select the virtual hub.
2. Click the Manage Virtual Hub button.
3. Click the Manage Cascade Connections button.
4. Select the connection and click the Edit button.
5. Click the Security Policy button.
6. Set "Filter DHCP Packets (IPv4)" = Enable.
7. Set "Filter DHCP Packets (IPv6)" = Enable.
8. Click OK.
9. Click OK again.
10. Toggle the connection offline and online.