You’re going to need a USB to serial RS232 cable, and either:
a) your HA machine connected directly to the Dax88 rs232 port. No ser2net needed. Your “port” in the configuration.yaml should be “/dev/ttyUSB0”. Or,
b) another machine, a Raspberry Pi in my case, connected to the Dax88 rs232 port. This machine needs to run ser2net. Your HA machine will essentially use this as a proxy to forward commands to the Dax88.