If some one else have trouble that the Nrf52840 Dongel do not mount using cdc acm on openwrt this will most likely help.
First number is the verdor id second is the product id, you will find the numbers in the sdk_config.h file.
echo '1915 521f' > /sys/bus/usb/drivers/cdc_acm/new_id
Cheers
/Jimmy