Have you restarted the system to ensure the changes are applied? I’m assuming you’ve already run update and upgrade.
Since this is an older PC, what baud rate did you configure for the port? Instead of testing with AAHMI, try using a basic terminal program like minicom to test the connection. You might also want to test a different port to rule out hardware issues.
It seems the Waveshare serial adapter uses either the FTDI or CH340 chipset, so make sure the appropriate driver is installed and functioning correctly.