I own a TP-Link Archer C7 AC 1750 v2.0 since some time and needed another access point, so I bought an almost identical used model, because I was happy with it (my first one is named "ARCHER C7(EU)", the second doesn't have the "EU" in the model name).
My second AP came with an older (snapshot) version of openWRT installed on it (it seemed to be reset to the default settings).
I immediately upgraded the firmware to version 18.06.4 r7808-ef686b7292 using the built-in upgrade function in luci (Firmware file was openwrt-18.06.4-ar71xx-generic-archer-c7-v2-squashfs-sysupgrade.bin)
After the upgrade, I couldn't log in using luci (ssh worked fine) and ended up resetting the device according to
The first attempt to factory reset via SSH didn't work, so I ultimately used the Failsafe Mode
Everything seemed fine, I configured the device, but after a reset, all my settings with the exception of the password were lost.
Whenever I change (and save) something in luci, it gets written to the config file (for example /etc/config/wireless), but after a reset, it is gone.
What could be the reason for this, and how do I fix it?