Yes if you have an ntrboot preflashed card that's the best option (you could've avoided trying frogminer altogether, since ntrboot is so much easier )I suppose then that I will look for another way to achieve my goal, since there does not seem to be a solution at the moment.
PS: Maybe I use the flashcart (ntrboot) method, because I have an R4i Gold 3DS Plus with ntrboot pre-flash (ntrboot already comes inside that flashcard by default)... (No, I better use an Ace3DS X, it seems more comfortable when using the ntrboot mode, hehehe)
In any case, thank you very much for your help.