Hello,
After some research, I think I understand why flash squashfs.bin images from the OEM web interface is not working.
When you upload a firmware to the webpage, it get the full size from uImage Data header and not the file size.
That mean, if the full OpenWrt firmware is 7 MB, in the uImage header will appear a size of ~1,8 MB, corresponding to the Kernel.
The main problem is that doing some hacks (set uImage header to 7 MB) WILL flash the whole image, BUT OpenWrt will supose that the Kernel is 7 MB (instead of 1,8 MB) and the boot will fail:
[ 2.861104] spi-mt7621 1e000b00.spi: sys_freq: 50000000
[ 2.872486] m25p80 spi0.0: mx25l12805d (16384 Kbytes)
[ 2.882661] 5 fixed-partitions partitions found on MTD device spi0.0
[ 2.895328] Creating 5 MTD partitions on "spi0.0":
[ 2.904883] 0x000000000000-0x000000030000 : "Bootloader"
[ 2.916770] 0x000000030000-0x000000040000 : "Config"
[ 2.927854] 0x000000040000-0x000000050000 : "Factory"
[ 2.939132] 0x000000050000-0x000000ff0000 : "firmware"
[ 2.951865] random: fast init done
[ 2.991139] 0x000000ff0000-0x000001000000 : "Second_Config"
[ 3.004283] libphy: Fixed MDIO Bus: probed
[ 3.076818] libphy: mdio: probed
[ 4.483005] mtk_soc_eth 1e100000.ethernet: loaded mt7530 driver
[ 4.495632] mtk_soc_eth 1e100000.ethernet eth0: mediatek frame engine at 0xbe100000, irq 17
[ 4.515354] NET: Registered protocol family 10
[ 4.525766] Segment Routing with IPv6
[ 4.533132] NET: Registered protocol family 17
[ 4.542097] 8021q: 802.1Q VLAN Support v1.8
[ 4.553483] hctosys: unable to open rtc device (rtc0)
[ 4.564464] VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
[ 4.579369] Please append a correct "root=" boot option; here are the available partitions:
[ 4.596007] 1f00 192 mtdblock0
[ 4.596014] (driver?)
[ 4.609009] 1f01 64 mtdblock1
[ 4.609015] (driver?)
[ 4.622015] 1f02 64 mtdblock2
[ 4.622021] (driver?)
[ 4.635030] 1f03 16000 mtdblock3
[ 4.635036] (driver?)
[ 4.648030] 1f04 64 mtdblock4
[ 4.648035] (driver?)
[ 4.659316] mtk_soc_eth 1e100000.ethernet eth0: port 3 link up
[ 4.672633] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[ 4.690729] Rebooting in 1 seconds..
Not sure if is possible to force OpenWrt to ignore uImage header data size...