Some time ago there was a problem with the openwrt installation since there are several sets of partitions where openwrt can be installed on this device (mmcblk0p3-mmcblk0p4 and mmcblk0p7-mmcblk0p8)
Initially it was only possible to install to the mmcblk0p7-mmcblk0p8 set and to be able to install correctly you had to run "change_boot_partition.sh".
It also happened that if it was installed in the set mmcblk0p3-mmcblk0p4 openwrt would not update and you had to change the boot partition for it to update correctly.
Currently these problems do not occur as the installation and sysupgrade scripts have improved and these issues no longer exist that I know of.
the NBG7815 wiki is a bit outdated in this regard.
This point 2 should say something like:
If you already have a very old version of openwrt for this device, to upgrade to a newer version of openwrt you can use sysupgrade or LuCi “Flash new firmware image”. It may be necessary to run https://github.com/itorK/nbg7815_tools/blob/main/change_boot_partition.sh to change the boot partition if it does not update correctly.
And if you are going to upgrade to a newer version don't forget to run wpad stop and wait a few seconds before running the sysupgrade command. (there is a problem for all ipq8074 devices with the sysupgrade process)
If there is any error in what I have exposed, someone with more knowledge correct it