So I am seeing something similar to this on my AC5300 node. Main router is also AC5300; ethernet backhaul mode enabled. Problem has occurred over last several Merlin beta / release builds; currently both running 386.2 Alpha 1.
I was using SmartConnect with both 5g bands to try and keep AC-capable devices on 5G-2, while allowing mixed N/AC on 5G-1. At that time I was having issues with devices connected on 5G-1 suddenly not being able to reach anything on the network, still looked connected, no errors in logs, but could not connect to anything on my local network or Internet.
A couple of release ago (like 386.1 beta 4 or 5) I decided to remove SC from the equation and set up different SSID and fixed channel allocations for all 3 radios, and manually assigned devices. 5G-1 for mixed N/AC and 5G-2 for AC only devices. In trying to troubleshoot I had all the other devices connected to 5G-2 (AC) and used just my iPhone 12 to see if I could figure out what is going on with the 5G-1 band. I cannot reproduce this on the main router.
I had dirty flashed through all those prior releases. So the other night, with 386.2 A1 already in place, I removed the node, WPS reset the node, WPS reset the main router, formatted JFFS on reboot, did GUI restore to factory defaults (with check to remove all other data) and then set up manually from scratch; no scripts or restore or nvram set in sight. Lastly, I re-added the node and enabled ethernet backaul again.
Was very hopeful this would resolve the problem but alas no. I am now thinking it might be hardware specific but am not sure if there are any diagnostics that can be run on the hardware by SSH'ing to the node. Some of the WL commands I have read about and have tried for radio power, etc. do not seem to be implemented or give no output. Would appreciate any input on that front.
Other option would be to switch the two identical devices (make node into router and router into node) and see if the issue follows the unit or the role. Obviously if it follows the device then hardware, and if it follows the role then it is software / config.
Any other ideas ?