Thank you very much to all.
I've finally managed to solve the problem.
First of all, to save some time to people who might try this, which didn't work for me:
The tool seems to work for v7, but it only extracts .idx and .dat files from the backup file, which are still codified (and it doesn't look like the tool is able to decode such files).
I even tried with another tool (
https://github.com/0ki/mikrotik-tools) which seems to be able to decode the idx and dat files, but this is an old tool that needs Python2 (instead of 3. First problem) and when I made it work (the instructions are almost non-existing), still it didn't seem to be able to decode everything, so I would only get partial information.
Finally, I succeeded by trying something inspired by this message:
Indeed. The only thing you can try to recover it is loading the backup on the new device, then export it, netinstall the new device and use the export to manually recover some settings.
And you should only attempt that when you are familiar with netinstall and winbox connecting using MAC address.
If not, better start from scratch with the new router and forget about the backup.
So, instead of using a real router, I created a Hyper-V virtual machine with routerOS, restore my backup there... and it worked!
Instructions for the virtual machine are here:
https://help.mikrotik.com/docs/pages/vi ... d=18350234
The only problem is that after the backup was restored, I didn't have network connectivity, but I was able to export the configuration to a file and copy it to a second virtual disk drive attached to the virtual machine with something like this:
/disk print
/disk format-drive (PUT_HERE_THE_DISK_NUMBER) file-system=fat32 label=flashdrive
/export file=disk1/exportfilename
Thanks!