airtel fiber on bridge mode ?

Possible to enable this bridge mode thing on a zyxel PMG5617-R20B router? Airtel pushed some update to the router few weeks ago and now I cannot change dns. There was an Apply button next to the dns field but they removed it now. What's their email btw for requesting to enable bridge and shit?
 
But I thought when bridge mode in enable the airtel ont won’t be connected to internet at all because it only works as modem . That means airtel onts WiFi will also not work .
It should not. Check if you're behind double NAT. Go to whatismyip.com and see if that matches the WAN IP of your non airtel router.