The Guest DNS filtering requires you to use a different IP from the one used for all other users. If other regular users share the same IP, the guest network policy will take precedence, diluting the level of the filtering application.
Securly has set up a separate set of DNS servers that only handle Guest Network traffic. So, directing traffic to those DNS servers via an SSID or VLAN is the easiest way to get your Guest Network up and running.
The Guest Network DNS Servers only respond to registered IP addresses so you need to make sure whatever external IP address that traffic will come to Securly via is registered to your account.
The next part of getting the Guest DNS filtering up and running is to just set up a Guest Network policy from the Policy Editor.