Solved Issues with Wireless WiFi clients - can't even ping

[AbominableSlav]

Hello, everyone. Please excuse me if it's not the right category, it's my first post.
I'm having an issue with my RT-AC88U on 386.7_2.
After setting everything to my liking (including smart connect rules, etc.) I now have the following issue:
Wireless clients can't communicate with one another. Not even a ping goes through. Wireless to wired is fine (I can ping and access my NAS for instance) - but wireless to wireless? No luck.
Why do I need that? Well, my wireless printer is one example, another is KDE Connect (an app I use to control my PC and laptop from my phone).
I know there's a "Set AP Isolated" setting in Wireless->Professional tab and it's disabled. Also tried enabling-disabling it.
I know the IP configuration is sound - I can ping both devices from Network Tools on the router.
Neither my phone nor my laptop can ping one another though.
Is there a setting I'm missing here? Has anyone had this issue before? I would appreciate any help...

 

[ColinTaylor]

Is there a difference in connectivity if both of the wireless clients are connected to the same AP as opposed to being connected to different bands (i.e. 2.4 GHz to 5 GHz)?

I'm assuming you're not using guest networks.

 

[AbominableSlav]

Is there a difference in connectivity if both of the wireless clients are connected to the same AP as opposed to being connected to different bands (i.e. 2.4 GHz to 5 GHz)?

I'm assuming you're not using guest networks.

Thank you for your response!
I have fixed the issue just now.
So basically this thread has the right idea:

RT-AC88U Problem Communicating Between Wireless Clients

I'm running Asuswrt-merlin 382.1_2 (though I've noticed the issue over pretty much all versions I've run). Even with Set AP Isolated disabled, I'm having issues with communications between wireless clients. I've even enabled and disabled Set AP Isolated in case there was some stale configuration...

www.snbforums.com

But I failed to realize these changes don't happen when you apply, you need to reboot as well. (that's what made me think it's not a solution)
MU-MIMO and Airtime Fairness to OFF for both frequencies/bands and a reboot was what fixed this.