[Solved] OpenWrt on MT7621/MT7615N devices with 5GHz problems

Just to be clear, the only real issue is with the 5 GHz WiFi.

The ethernet routing performance (using arinc9's patches) is very good/excellent.
The MediaTek MT7621A is also used in MikroTik hEX RB750Gr3 and hEX S RB760iGS