Hi,
The HTTPS Persistence is used for sessions like online Banking where if the session is spread across different WAN connections you would have to re-login each time the web server sees the traffic coming from a different location.
In most cases the DNS failure can be solved by setting the WAN Health Check method to use a DNS Lookup of Google’s public DNS servers (8.8.8.8 and 8.8.4.4).
Below I have pasted links to Knowledgebase Articles that go into more detail of Outbound Policy and the Health Check mechanisms.
Outbound Policy: Understanding and Configuring Outbound Policy
Health Check: Health Check Mechanisms Against Link Failure
To force traffic to fail over to the second WAN connection you can change the Outbound Policy to use Priority. Priority 1 will send traffic through this WAN connection and when it fails Priority 2 will take over.