My apologies for the kick.
@ninousf and @_AndreaS_ , did you end up solving it? Running the 16MB build of ESPEasy here, I'm trying to use the MOD-RS485-ISO with HW serial1 on GPIO's 36 (Rx) and 4 (Tx).
So far no luck.
The way to test UART of ESP32-PoE-ISO is pretty simple, connect RX and TX (with a jumper wire with female-female connectors) and open terminal and type something and see if there is echo (e.g. what you type gets printed back). If there is echo - the UART is fine and the issue is somewhere else.
I think the issue is just wrongfully using MOD-RS485-ISO, it is not a basic bare convertor. It has a main microcontroller and own firmware, and while it can be used with Modbus or similar, it is meant to be used with own firmware and commands.