Cannot Install Any New Updates On 7.2.9

Nov 4, 2016
after I upgraded to Nougat using the 7.2.9 build, now I cannot flash any new updates.
I tried to unlock the bootloader using the unlock tool. It succeeded, but on subsequent upgrade retries the phone reboots with the same old 7.2.9 version.
If I try the unlock tool again, it says it is not necessary to unlock (already unlocked?), but I still cannot upgrade.
The current version (7.2.9) heats horribly and depletes the batteries in half a day.
Any ideas what else can I do?