I tried to use the US stock firmware in the Web Dialog that the router offers via 192.168.0.1 whereas the file between EU and US is 100% identical:
and the update started... so it just parses the filename?
product-info:product_name:Archer C7
product_ver:5.0.0
special_id:55530000
[Error]sysmgr_cfg_checkSupportList(): 1023 @ specialId 45550000 NOT Match.
Firmware supports, check OK.
Firmware Recovery check ok!
upgrade_filecheck ok
HTTP upload is done! Upgrading...
do http upgrade
Firmware Recovery file length : 15567490
Firmware process id 2.
handle_fw_cloud 146
Image verify OK!
Firmware file Verify ok!
product-info:product_name:Archer C7
product_ver:5.0.0
special_id:55530000
[Error]sysmgr_cfg_checkSupportList(): 1023 @ specialId 45550000 NOT Match.
Firmware supports, check OK.
Firmware Recovery check ok!
set integer flag to 0.
######################################################################
######################################################################
######################################################################
##################################
Done.
set integer flag to 1.
Firmware Recovery Success!
HTTP ugrade is done! Rebooting...
However, after the update my device appears as AC1900
So I recovered back to the OpenWRT image
tftp 0x81000000 18_06_2_squashfs-sysupgrade.bin
erase 0x9f0c0000 +800000
cp.b 0x81000000 0x9f0c0000 0x800000
reset
and but it boots by itself without any manual 'boot' or 'bootm'.
Something must got repaired by flashing the stock firmware, even if the device appeared as AC1900 at the end.
@mk24, thank you for your suggestions! It was helping to get a little bit closer to OpenWrt.
I hope this story is done now, I wrote down my entire tests and hope it will help others to get alike problems solved (I found many post with bricked devices).