RBR750 blocking Minecraft servers for Android

I've had my ups and downs with this router, but usually I'm able to sort it out. This one's a head-scratcher, though.

In trying to help my son troubleshoot a Minecraft issue, I installed the app on my phone as a test case. I've never really played Minecraft, never had it on my phone, etc. - it's a clean install. We both use Android devices.

The issue we're facing is that we can't connect to the Featured Servers shown in the app. This feature apparently used to work. It does work on my phone if I turn off Wi-Fi and use my mobile network (and similarly, if I set up my phone as a hotspot and let my son connect his tablet). So at least I know it's an issue related to the router and/or my ISP. Having just factory reset the router about a month ago, I figured maybe I had missed a setting and inadvertently caused a problem. But for the life of me, I can't figure out what I might have done that's causing this problem.

Even more curious, while I was toggling various Orbi settings and applying them, if I tried the Minecraft featured servers at just the right time during the minute or so when the router was applying new settings, it would work. Just for a moment, though. Something going on during that settings refresh allows it to work, but by the time the router finishes applying the settings it's blocked again. But that also tells me it's the router, not my ISP.

Any suggestions for settings to check or other fixes? I'm using FW 4.6.8.2_2.1.9, which I know had some issues.

Thanks.