There was a sale on MR8300s, so I went out and got one of my own.
I used fwupdate.html to install OpenWrt 19.07.3 r11063-85e04e9f46 to it, and things appear to work, but the device has gotten knocked offline three times in the last four days. Both web and SSH access stop working. No obvious diagnostics appear to be retained between boots, so short of opening the thing up and attaching a serial debugger I'm not sure what to do beyond live with it.
 I was mostly looking in
/var/log, which may well be the wrong place to look.
First, try using EA8300 recovery boot sequence to go back to backup OEM firmware :
A little past half way though the page:
The EA8300 is a dual firmware device. ie. There are 2 partitions and Linksys firmware is copied to both partitions at the factory. When you install/update the Linksys firmware, or install/update OpenWrt, the new firmware is always written to the other partition. Upon restarting the EA6350v3, it will subsequently try to boot from the newly installed firmware image from other partition.
If the device fails to boot after install or upgrade, whilst the unit is turned on:
- Wait 15 seconds
- Switch Off and Wait 10 seconds
- Switch on
- Repeat steps 1 to 3, 3 times then go to 5.
- U-boot will switch back to the last working firmware - you should be able to access your router on LAN.
The above method can be used to toggle between the two firmware.
Warning: If you install/update OpenWrt more than once, both partitions will be overwritten with OpenWrt firmware, and you won't be able to revert back to Linksys OEM firmware using above method.
If this works, reflash OEM firmware twice, to make sure you kill OpenWRT and have both primary and backup as OEM versions, this will allow you to start from scratch and still have the OEM version as backup.
Once you reflash to OpenWRT, enable SSH access only from LAN, as the knock offline could be due to attempted SSH access from WAN. Just a thought here...
Always make sure you don't erase the OEM backup, it's your way out of a mess, where you can regroup back to known working state, allowing you to start over from scratch. I already put my self into unresponsive device state with an EA8300 and managed to recover entirely back, reflashed to double OEM firmware and upgraded successfully to 19.07.3 thereafter.