Hello,
I would like to experiment a little with the GPL firmware "GPL code: DIR-842 C3 FW 3.14EU" found at https://tsd.dlink.com.tw/, and if it does not work out then restore the official firmware, which can be downloaded from the d-link website. However, at the end of the readme file there is a strange phrase
2) Once you upgrade to GPL firmware, you will never be able to upgrade back to formal firmware.
What does it actually mean and what becomes different after trying another firmware?
FWIW, that GPL firmware corresponds to the currently installed firmware on my router.