Success - I am now running snapshot of v20.
I did upgrade to 19.07.04 .
I ran
fw_setenv boot_openwrt bootm 0x9F120000
fw_setenv bootcmd run boot_openwrt
but
fw_saveenv
does not exist.
I tried rebooting the deivice, but it never came up. I guess, that the changes was saved with fw_setenv, even without fw_saveenv
So I continued installing the snapshot with tftp using recovery method.
My NBG6616 is not running snapshot of version 20
I have tried opkg update;opkg install luci, but it fails because rpcd_2020-05-26-078bb57e-1_mips_24kc.ipk was missing in the repository.
I added
src/gz snt_base http://ftp.snt.utwente.nl/pub/software/lede/snapshots/packages/mipsel_24kc/base
to /etc/opkg/customfeeds.conf
Then opkg update;opkg install luci
was working