Draytek Vigor3910 Series User Manual page 386

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12. Enter the mobile phone number to receive the SMS message.
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
Due to security concerns, the browser might warns that it cannot
verify server identity, the clients would need to tap "continue" before
they can proceed to portal.draytek.com.

The client might not be able to access "portal.draytek.com" if this
domain name is resolved by a DNS server on LAN. If so, set up LAN DNS
to make sure the domain name will be resolved to the router's LAN IP.
Vigor3910 Series User's Guide