Well I persevered and managed to make DSATM work.
Thanks to archive.org I found an old version of the program and here I am.
GUIDE: HOW TO play NDS games in WIDESCREEN on MelonDS for Nintendo Switch
1: First of all you need a legal dump of your .nds game, then go here
https://gbatemp.net/threads/widescreen-cheats-for-ds-games-on-3ds.543212/#post-8712884 and find the cheat code you need.
2: Download DSATM.rar on this link
https://ibotmodz.net/forum/index.php?app=core&module=attach§ion=attach&attach_id=4085 (this is a previous version of the program, the one available for download here on GBATEMP doesn't work)
3: Unzip DSATM.rar in a folder, and in it create an empty file called DSATM.ini. In the same folder create a .txt file with the cheat code in it.
4: Run DSATM.exe, load the .nds game, the .txt file and name the output.nds file, then click on Autopatch.
THE END: Now you can put the widescreen.nds game in the switch/melonds folder or wherever you want in your SD card, and run it through MelonDS.nro (remember in MelonDS settings to change the sizing on Top only, and the Aspect ratio on 16:9, with the Nearest filtering)