Thx @FluBBa it works !I know I see it when testing home-brew stuff, but everything else works for me... =(
The ".lnx" files should include a header which is needed still, there are also ".lyx" files which doesn't (shouldn't) have a header.
I don't know if that helps.
here you can test if your rom is in ".lnx" format.
https://atarigamer.com/lynx/lnx2lyx
I successfully launched Batman Returns!
If anyone is interested in the method here is what I did:
1. find and download lynx games in .lyx format (don't ask me, it's very easy to find on Google)
2. convert games here:
https://atarigamer.com/lyxconverter
3. download the .lnx file + header
And there you have it
I did this because my original .lnx files, the first ones I tested, were not recognized in the lnx2lyx converter