Hello,
It's hard to say what the exact issue could be, as this firmware is no longer supported and it's a custom GPL build, however, to overcome the issue a user script could be used.
User scripts are run as soon as the device finishes booting up. In your case, you'd need to restart the Modbus service. This can be done using this command:
/etc/init.d/modbusd restart
/etc/init.d/modbus_serial_master restart
/etc/init.d/modbus_tcp_master restart
This will restart basically all Modbus-related services. If this does not help, UCI could be used to change the values, if by default it's set to 0. The command would look like this:
uci set modbus.modbus.device_id='1'
uci commit modbus
/etc/init.d/modbusd restart
This should set the correct values. Let me know if it helps!
Best regards,
DaumantasG