The new firmware is working fine so far.
Before flashing that to mtd, it's necessary to add a proper u-boot header.
Just using u-boot/tools/mkheader.py like that:
python2 mkheader.py 0x44000000 0x13 AQR-G4_v5.5.6-AQR_CIG_WIFI_ID44858_VER1745.cld aqr_5.6.mbn
The file aqr_5.6.mbn has now the right header and can be flashed to the /dev/mtd10 partition.