Smart Connect issues across interfaces

[jay42k]

On my LAN, I cannot use filesharing, ping devices, etc. when using Smart Connect and devices reside on different interfaces. Not sure what is happening. For transparency, I am 100% Apple products (Macbooks, iPhone, iPad, etc). Examples if using two Macbooks:

• Smart Connect ON and devices are on SAME interface (e.g. both on 5GHz-1): Devices perform as designed. I can ping from one macintosh to another. File sharing works.
• Smart Connect ON and devices are on DIFFERENT interfaces (e.g. one on 5GHz-1 and one on 5GHz-2). Pings do not work. File sharing does not work.
• Set up GUEST network and connect both devices on the same GUEST network (which puts both devices on the same interface): I can ping from one macintosh to another. File sharing works.

In all cases, internet access across the WAN works beautifully. No issues.
This is a simple network. I connect all devices on one SSID with Smart Connect.

Is this something with Apple / Macs? I have similar issues with my iphones doing app syncing to my Mac. If the iPhone is on the same interface as the Mac, the app sync works beautifully. If they are on different interfaces, the app sync does not work.

Using Asus Merlin 380.59 on Asus RT-AC3200