SSL VPN uses TLS and SSL protocol, which is widely used by all the browsers for HTTPS connections. Therefore, even if there is a firewall, as long as HTTPS traffic is allowed, SSL VPN will be able to work through.
DrayTek dynamic DNS service (DrayDDNS) offers each Vigor Router a hostname on the domain drayddns.com. It's free and only needs to renew once a year. With DrayDDNS, VPN clients can access to the router by a fixed hostname of your choice.