Please see DrayTek’s VPN Application notes for general information on configuring VPNs.
The minimum requirement is for a Dynamic DNS at the Dial-In location. At the Dial-Out end, in section 2. Dial Out Settings please enter the dynamic DNS into the field titled “Server IP/Hostname for VPN”.
When creating an IPSec VPN you will notice under VPN and Remote Access/LAN to LAN/Dial-in Settings that IPSec requires the “Peer VPN server IP” or “Peer ID” fields; and that the instructions on the DrayTek website generally show examples with static IP Addresses at both ends.
Fortunately the IPSec Tunnel (aggressive mode) example shows an email address as the Peer ID. The important thing is that at the branch office (i.e. Dial-out end), to press the [Advance] button and enter the same value in the “Local ID” field.