AU plug-ins not showing in GarageBand on MacBook Pro

GarageBand - downloaded AU plug-in not showing up on MacBook Pro.

Hi - I'm running 10.4 GarageBand on Sequoia on my 2018 MacBook Pro. Trying to d/l trusted 3rd Party AU plug-ins. Installation steps will say installation was successful, but it won't show up in the plug-ins area of the Smart Controls even after quitting and reopening GarageBand. I've also tried restating the Mac after downloading. I have also checked that the "Allow Applications from" in Privacy & Security will allow downloads from the AppStore & known developers. Also made sure the "Enable Audio Units" is checked in the Audio/Midi setting of GarageBand. Advise? Thanks! : )