WAN active-active configuration not working

Question

Hello Experts, 
 
 i have 2 WAN lines configured active-active with weight =1 for both. Line_1=30 MBPS and Line_2=10 MBPS. The problem is when Line_1 goes down the Line_2 is not coming up. 
 we loose internet but strangely ping works to google, I can browse google and search but when i click on any search results it doesn't work neither it works when I try to browse any site or outlook. 
 It works only when I change my local machine DNS to 8.8.8.8. 
 My DNS setting are 2 global DNS and 3rd is the ISP DNS. 
 
 SOPHOS support says its the problem with the internal DNS (LOCAL LAN). But i am wondering how it works with Line_1 with same setting in internal DNS? 
 
 Please help, Thanks

Aditya Patel · Answer

Hello Shrikant, 
 While you are dealing while using multiple WAN connections, it is always advisable to have a DNS server pointing to your AD server or Sophos XG. 
 Reason-> The system will always try using the primary server host address and if that fails it fails. If you are using Sophos XG, then you may add up to 3 DNS servers. I would recommend first 2 from both your ISP DNS1> WAN1 DNS2>WAN2 and keeping the third DNS3> Public 8.8.8.8 Under DHCP settings make sure that SOPHOS XG LAN IP is is set to primary DNS server for your client machines. 
 If you have AD server, you may configure additional DNS forwarders and while keeping some of them in Public DNS as well.