Problem Description:
Site to Site IPSec VPN between SonicWall Security Appliance and DrayTek doesn't list all IPSec Security Associations (SA) advertised to remote DrayTek Firewall under VPN status on SonicWall Security Appliance.
Resolution:
Enabled the option "create phase2 SA for each subnet" on the DrayTek firewall in order resolve this issue. follow the KB to enable the option:
https://www.draytek.com