I don’t have a FlatMan panel, but built my own using an LED panel from Amazon and an Arduino Nano. However, the programming uses the protocols for the FlatMan series and when I use it with SGPro, I connect it as a FlatMan panel. I wouldn’t claim to be any kind of expert on Arduinos, but I have spent many frustrating hours trying to get them to talk to PCs over the serial connection, and this may be where your problem is. The way to check this would be to try to get into a conversation with the FlatMan panel (in these times of Covid, any conversation is welcome!) using a serial console - one of the most common ones is PuTTY. You need to know what com port your FlatMan is on and you can open up a line of communication with it using (e.g.) PuTTY (which looks like this in my case on com port 7). This will open up a terminal window into which you can type commands. For instance, if you type “>S000” (without the quote marks) followed by a return, if all is working correctly, you should get a response on the terminal of “*S15000”. The 15 in the string indicates that it is the FlatMan-XL. If you get no response, then either the serial settings are not right or there is a problem with the panel.
On the assumption it is a serial connection issue, then the first thing to be sure of is that the baud rate is 9600 with 8 data bits, 1 stop bit and no parity. Assuming that is all correct, then what I’ve found is that the Flow Control can be important and so it might be worth changing that - you can do that in PuTTY from the Serial category. Normally the control is “XON/XOFF” but other options are “RTS/CTS”, “DSR/DTR” or “none”. You could try one or all of the others to see if you can get a response.
Clicking on the links in the text should show a screen-grab image which might be helpful.
Good luck troubleshooting the problem.