- It's possible that you use wrong vendor firmware, isn't it? Try older/newer version.
- You can try prepare new image from vendor firmware for flashing from openwrt command line.
Vendor firmware file contain all information for this procedure. Flash layout:
partition fs-uboot base 0x00000 size 0x20000
partition default-mac base 0x20000 size 0x00200
partition pin base 0x20200 size 0x00100
partition product-info base 0x20300 size 0x00200
partition device-id base 0x20500 size 0x0fb00
partition os-image base 0x30000 size 0x100000
partition file-system base 0x130000 size 0x6a9400
partition soft-version base 0x7d9400 size 0x00100
partition extra-para base 0x7d9500 size 0x00100
partition support-list base 0x7d9600 size 0x00200
partition profile base 0x7d9800 size 0x03000
partition default-config base 0x7dc800 size 0x03000
partition partition-table base 0x7df800 size 0x00800
partition user-config base 0x7e0000 size 0x0c000
partition certificate base 0x7ec000 size 0x04000
partition radio base 0x7f0000 size 0x10000
And vendor firmware layout:
fwup-ptn product-info base 0x01000 size 0x00115
fwup-ptn os-image base 0x01115 size 0xffdb8
fwup-ptn file-system base 0x100ecd size 0x69363f
fwup-ptn soft-version base 0x79450c size 0x00059
fwup-ptn extra-para base 0x794565 size 0x0000b
fwup-ptn support-list base 0x794570 size 0x00055
fwup-ptn profile base 0x7945c5 size 0x02e1d
fwup-ptn default-config base 0x7973e2 size 0x02a34
fwup-ptn partition-table base 0x00800 size 0x00800
So you can split vendor firmware file to get kernel (os-image) and rootfs (file-system) files. Then pad kernel file upto os-image partition size (partition os-image base 0x30000 size 0x100000) and append rootfs. This new firmware file may be flashed from openwrt command line with sysupgrade or mtd command.
P.S. To split vendor firmware file you can use tplink-safeloader utility from current openwrt trunk.
P.P.S. For example I use file ArcherC6v2_ru-up-ver1-3-0-P1[20200508-rel63078].bin