Ignition 7.9 with serial client gateway on Windows Server 2019

Not in v7.9, no. IA switched to Fazecast's library in v8+, presumably because Serialio wasn't keeping up with modern platforms.

v7.9 won't be patched to fix this. Downgrade your Windows server if you really need this. Consider switching to Modbus RTU over TCP with a Moxa network serial converter.