Failover Doesn't work with Unifi Equipment

My primary Internet connection is with the BGW320-505 setup in Pass-through mode connected to my Unifi Dream Machine Pro.  (UDMP)

My secondary Internet connection is with Xfinity cable modem.  This is a great secondary option for me as it only costs me $20 a month. 

The issue is, I've had three AT&T outages due to outside fiber cuts in my area in the last 4 months.  My UMDP does not fail over to my Xfinity connection unless I rip the cable out of my BGW320-505's ethernet connection to my UDMP or change the port configs in the Unifi software.  After spending two weeks with Unifi Support, they said the issue is the AT&T BGW320-505 hijacks the DNS and responds to DNS queries.  

According to Unifi Support, this is how they determine uptime below.  The issue is that the AT&T BGW router is responding to 8.8.8.8 and 1.1.1.1 DNS queries even if I simulate a fiber cut (by yanking out fiber cable from the router).   Their support said there is nothing more they can do and I should take this up with AT&T Support.   I tried leveraging packet filtering to perhaps block all DNS queries to 1.1.1.1 and 8.8.8.8 but there doesn't seem to be a way to block DNS queries (I even tried UDP and dest port 53) and it won't block that traffic.  Open to suggestions on this. 

The UniFi Gateway would report the Internet status as down if the Internet detection mechanism detects a very high latency/packet loss on its WAN interface. 20 pings to the internet verifications server and DNS requests to 8.8.8.8 & 1.1.1.1 are sent every minute and if there is >50% packet loss or >1500 ms latency is observed in the ICMP and DNS responses, the Internet connection is considered as down.