You are assuming correctly. Each of those is a byte, so Write a Byte. Do use the decimal equivalents, because Unreal only understands decimal in its node fields.
Right after you Write a Byte, if you call Read Bytes, does the array comes empty? Did you try to delay before reading the device’s answer? Maybe you’re just expecting the answer too soon?
I’d try it with a Read Byte on Tick for a while after sending a command. To see how long it takes to answer.
I recommend using Flush before sending a command, just to make sure there’re no previous bytes in the read stream waiting to be read. Because that could look like a command answer, but would just be stuff that was sent before your command.