@hnyman -- thanks a lot! this worked. I followed the procedure with small modification found in this post:
I loaded 19.07 initramfs, booted from it, and then sysupgrade -n. later I was able to flash version 21.02 via web interface and it also worked.
I have a few questions though.
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is my understanding right that once the procedure completed with success further openwrt upgrades can be done via web interface without issues? or maybe this is not 100% reliable procedure?
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if this procedure may not work in future what would be more reliable approach?
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with original procedure the booting stopped at "Starting kernel..." message. then I followed another advice to use tftpboot 50000000 and it worked. how to figure out this "magic number" 50000000? is it arbitrary? or does it depend on anything?
thank you
geos