I think the problem is that as a hashtable it is not in JSON format.
Here is what is stored for the variable but that is not the right format.
PS > $SSIDConfig
{"enabled":"True","ipAssignmentMode":"NAT Mode","encryptionMode":"wpa","wpaEncryptionMode":"WPA3 Transition Mode","name":"Test","authMode":"psk","psk":"TestTest1","lanIsolationEnabled":"False","bandSelection":"Dual band operation"}
Should look something like this:
{"Attributes":[],"File":null,"IsFilter":false,"IsConfiguration":false,"Module":null,"StartPosition":{"Content":"{name:Test,enabled:True,authMode:psk,psk:TestTest1,encryptionMode:wpa,wpaEncryptionMode:WPA3 Transition Mode,ipAssignmentMode:NAT Mode,bandSelection:Dual band operation,lanIsolationEnabled:False}","Type":19,"Start":14,"Length":205,"StartLine":1,"StartColumn":15,"EndLine":1,"EndColumn":220},"DebuggerHidden":false,"Id":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx","Ast":{"Attributes":[],"UsingStatements":[],"ParamBlock":null,"BeginBlock":null,"ProcessBlock":null,"EndBlock":{"Unnamed":true,"BlockKind":130,"Statements":"name:Test,enabled:True,authMthMode:psk,psk:TestTest1,encryptionMode:wpa,wpaEncryptionMode:WPA3 Transition Mode,ipAssignmentMode:NAT Mode,bandSelection:Dual band operation,lanIsolationEnabled:False","Parent":"{name:Test,enabled:True,authMode:psk,psk:TestTest1,encryptionMode:wpa,wpaEncryptionMode:WPA3 Transition Mode,ipAssignmentMode:NAT Mode,bandSelection:Dual band operation,lanIsolationEnabled:False