Justin Case someone wants to revert back to Netgear's Router Firmware, which took some tweaking and trying, for me.
This worked for me, finally. From OpenWRT - (openwrt-19.07.1-ramips-mt7621-r6220-squashfs-factory)
Used nmrpflash - https://github.com/jclehner/nmrpflash
Used Windows 10
Set router IP to 192.168.1.1
Ethernet card of laptop 192.168.1.2/24 Def Gw 192.168.1.1
TP Cable to LAN1 on R6220
Router turned on, waited for "Advertising NMRP server on net5 ..." and turned off the router for 3 seconds, then on again.
C:..\Downloads\Firmware_Release>nmrpflash -i net5 -f R6220-fw.img -m 40:5d:82:cc:b3:68 -vv
net5: \Device\NPF_{F657D98F-FA7D-4DD2-A440-79B8A98D2F73}
Adding 10.164.183.253 to interface net5.
Advertising NMRP server on net5 ... | <-- Turned off router HERE, waited for 3 sec, then ON
Received configuration request from 40:5d:82:dd:c3:78.
Sending configuration: 10.164.183.252/24.
Received upload request without filename.
Using remote filename 'R6220-V1.1.0.86.img'.
Uploading R6220-V1.1.0.86.img ... OK
Waiting for remote to respond.
Received keep-alive request (249). (took some minutes for it to finish)
Remote finished. Closing connection.
Reboot your device now.
C:..\Downloads\Firmware_Release>_