I suspect this post: https://community.tp-link.com/en/business/forum/topic/241734 is talking about the same problem

I have noticed random DNS timeouts with the TL-R470. Things work well initially but after 5 minutes or so I start getting DNS timed out (Port Unreachable) ICMP messages coming back. My setup is two...

@terziyski thanks for those suggestions. Adding a static route lets me get to the modem ok now.

Unfortunately I'm still getting DNS Destination Port unreachable errors, no matter what I put in the Optional fields. I will post this problem on the business forums. Getting back to the VR400, I...

I did put the DNS values the modem gets from my ISP in the optional fields, but now specifying 8.8.8.8 and 8.8.4.4 in the Optional fields, things are looking good. thanks for your help

The DNS timeout problem is definitely in the load balancer. Using Wireshark I see DNS ICMP packets coming back from the load balancer saying: Destination Port unreachable If I change my desktop to...