Back up configuration , there is no migration path past 2 points:
v17x is needed to support next sysupgrade format
then v19x architecture change to ath79, config wiped.
Those both need to be installed 17.x.0 -> 17.x.latest
i.e 5x upgrading still losing config.
You need to type configuration back via luci web.
People who don't own the same device are unlikely to know more about it than what's written in the Device Page. You have the info about installation methods there.