"Using" block doesn't dispose the object explicitly, so it is not recommended.
Explicitly disposing the object after using it, as mentioned above, is the one recommended in EMDK for .NET.
This fact has been mentioned in the section "EMDK Programming" available under the section "Programmer's Guide" in the help file.

FusionInterfaceOpenEx is the native/C API invoked by the .NET class library Symbol.Fusion when opening the connection to the underneath Fusion API.
I'm assuming that you have properly installed all the required files on the device including all the relevant .NET class library files and these native/C DLLs.
Symbol.all.arm.cab installs all these files, so if this .cab has been deployed already on the device, then it may have already taken care of this part.

Anyway what's the exact failure you're getting related to FusionInterfaceOpenEx ?.
One thing I can think of related to Statistics Mode opening which would be resolved upon rebooting the device is the issue of reaching the maximum statistics mode open sessions allowed by the underlying driver (only one Command Mode session is allowed at a time, but multiple Statistics Mode sessions are allowed up to a maximum limit. After reaching the limit, further attempts would fail.). This situation would arise if you have not disposed the WLAN instances created at the .NET level.
So please check the entire code and add code to dispose the WLAN instances after use.

i.e.
myWLAN.Dispose();
myWLAN = null;

Let us know if this resolves the issue.
If not, please mention the complete failure you are getting (in case of OperationFailureException, you may specify both the exception message and OperationFailureException.Result property which may provide the information on the failure occurred underneath.).