Yes this works when I’m having issues.

Okay, gotcha. In that case, my guess is that the most-likely issue is with the DNS daemon that OpenWRT is using, which is most-likely dnsmasq.

You could confirm that your DNS configuration on the LAN hosts is actually using OpenWRT as the DNS server if you wanted

I could walk through that, if you're not sure. But from a test-when-it's-happening situation, I'd probably:

• Install dig on the Mint box, if it's not already installed. That's a DNS diagnostic program that doesn't use your system resolving mechanism, talks directly to DNS, so it cuts out any other factors. It'll be in the bind9-dnsutils package.

    $ sudo apt install bind9-dnsutils
  

• Install dig on the OpenWRT box.

    $ opkg update
    $ opkg install bind-dig
  

• Confirm that you can successfully run it on both the OpenWrt box and your laptop when there are no problems (your router's LAN address probably being something like 192.168.1.1):

    $ dig @your-router-LAN-address www.google.com
  

• Confirm that you can successfully run it on both the OpenWrt box and your laptop when there are no problems using whatever outside DNS server you have the OpenWrt box set to use (assuming 1.1.1.1 here):

    $ dig @1.1.1.1 www.google.com
  

• Next time you have problems, repeat the above. If both the dig on your OpenWrt box and on your laptop using the OpenWrt DNS server now fail, then it's probably pretty safe to say that it's the DNS server on the OpenWrt box at fault. If just the one on your laptop fails, then it's going to be a communication issue between your LAN box and the OpenWrt DNS server.

Assuming that the problem is the OpenWrt DNS server, and assuming that it's dnsmasq, to troubleshoot further, I might try looking at its logs, or installing tcpdump on the OpenWrt box and running that, something like $ tcpdump -nvvv -i any udp port domain. That should let you see both the incoming DNS queries going to the OpenWrt DNS server as well as the queries that it is sending upstream (the -n will prevent tcpdump from itself sending DNS queries to look up IP addresses, which would generate more noise, the -vvv to show all possible information about the packets, and the -i any to show all packets on all interfaces). That'll let you see whether it's getting flooded with DNS queries and to see what queries it's sending to the upstream DNS server out on the Internet.

That won't alone solve your problem, but it'd be the next step I'd try to narrow down what's going on.

For whatever reason ISPs tend (at least in here) to be pretty bad at keeping their DNS services up and running and that could cause issues you're having. Easy test is to switch your laptop DNS servers to cloudflare (1.1.1.1, 1.0.0.1) or opendns (208.67.222.222, 208.67.220.220) and see if the problem goes away. Or even faster by doing single queries from terminal, like 'dig a google.com @1.1.1.1'.

If that helps you can change your router WAN DNS server to something than what operator offers you via DHCP. I personally use opendns servers, but cloudflare or google (8.8.8.8, 8.8.4.4) are common pretty decent choices too.