So I soft-bricked partition 1 while re-configuring as dumbAP. Cannot access with ssh or gui.
Booting from partition 2 I thought there will be an option in luci to overwrite partition 1 with sys-upgrade image. Or barring that ssh in and dd the image to partition 1. There is no such option in luci so that leaves dd.
Referring to the partition layout for a v2 wrt1900ac it is the mtd4 and mtd6 partitions that contain sys-upgrade firmware, yes? So I should determine current partition then dd the image to the other partition.
I am not ready to do this yet.. I want to research more. Be happy to hear other ideas about this plan though.
No need for dd and direct writing to mtd partitions.
Both OpenWrt and the Linksys OEM firmware always write to the other partition, switch booting to that one, and keep the current one as the fallback option.
If you have a currently working partition, then simply sysupgrade from that one, and the new firmware will get written to the other partition.
So after flashing a community build sysupgrade (david502c) I am again able log in to the 'bricked' partition. All settings are default. However, the kernel is old, 4.14.54.. iirc that was the last kernel version on that partition. Also, I un-tarred the sysupgrade.bin (the latest build from david502c) and that kernel is 4.14.91 which is as expected.
At this point though I think this conversation belongs elsewhere. I will open a new topic in David's thread and link this one. When time allows..