@zusofuz :
Unfortunately it is not the only issue: NEXX 3020 (MT7620) Wi-Fi issues - It is interesting that the disconnection problem stopped and then it came back.
I wonder if it was/is due to another router on the same channel. A router will normally still work with some interferences but the mt7620 driver is still not shining.
Are you using the 2.4G connection? Does the 5G work? What is the speed you are getting?
It would be useful if you posted the openwrt version you are using.
Can you try to update to the latest version to see if you have any changes: https://downloads.openwrt.org/releases/18.06.1/targets/ramips/mt7620/openwrt-18.06.1-ramips-mt7620-miwifi-mini-squashfs-sysupgrade.bin