Probably problem is with the repeater mode of AR750s with Wireguard turned on and connected with the main router.
Problem is not with the DNS as no custom DNS is defined.
AR750s stops and starts broadcasting SSID after every few minutes. I stopped the repeater mode in AR750s and connected it with PoE adapters to main router and so far problem seems to have been resolved.
Another user is also facing apparently the same problem.