I created a build for AX3600 and DL-WRX36 from the official openwrt
for the AX3600:
if a 1 part release from Robi was already flashed
uci set system.@system[0].compat_version=1.0
and then sysupgrade
^^ everything works 
for the DL-WRX36:
if a 1 part release from Robi was already flashed
I did the same as with the AX3600
but then had no more connection over the network
(only after a reset on the device)
^^ does anyone have the same problem?