Playing minecraft and network connection drops

ISP router ( --> 1st OpenWRT PORT WAN -- LAN -->
2nd OpenWRT box on 192.168.5.* network.
1st OpenWRT handles DHCP, DNS
Both OpenWRT boxes have the same SSID.
Kids and I are connected to 2nd box, me cable and kids wireless. Two machines are using blue stacks android emulation and the third is running from an ipad.
I can work on the same PC all day without any issues. But once we crank up Minecraft we can be playing normally and suddenly the apps shows lost connection. The win10 box that I am on, the system tray will show no connection. Ping to a DNS server will also show a request timed out.
If at all possible, what I was after is where to start to diagnose. What log files should I be looking at?
We play a few times a week so I might be slow getting back to the thread as replicating the environment takes all of us. Thank you very much.

You never describe how your 2nd OpenWrt router is connected to the 1st (logically), how they're specifically configured, is the 2nd OpenWrt configured as a Dumb AP or a 3rd downstream router, etc. It's possible that could be the cause...or anything...

You'll need to provide more info via configs regarding that.

You could start with the OpenWrt devices (e.g. during/immediately after the issues occurs).

Sorry for not contributing more info.
1st router LAN port cat6 cable to 2nd router LAN port.
2nd router is setup as a dumb AP.
After the problem occurs I have been through the webgui status -->> system log / kernel log but I am not seeing anything which I believe to be interesting.
Is there another set of log files I can go through?
Is it better to go through the bash prompt and tail a log file and see if I can the problem to reoccur?
Unfortunately for me, a router is a device I set up and then forget about so when it comes to fault finding I just have no idea.

What device are your openwrt devices?

The behaviour you’re referring to sound like a nat connection limit issue, which could either be in the wrt router or the isp device.

Can you put the isp device into bridge mode to eliminate it as a source?

Both routers are AC1200 Model R6120.
I will see what I can do to eliminate the ISP router from the problem.
Will get back to you on the ISP router.

The ISP router is HP620 and PPPoE Bridge is enabled.