As title says in this article you can download several different custom ROMs for very interesting rugged smartphone. So below you can see several different ROMs. Also if you are interested you can see here review for Nomu S20.
S20 Official Rom 1.0.2 – download link
Just remember you will need to flash 1.0.2 FW to get OTA with 1.0.4
Chamelephone Rom Edición OTA / Mdsdev 6.0 – download link
Also if for some unknown reason often flashing your Nomu S20 has no IMEI now, then you will need to use SN Writer tool to write IMEI. Here you can download required tools here.