Check your SSID settings on the "Access Control" page, if you are using any type of splash page, and if so, there could be a setting for Network Access Control that's enabled to check clients for AV software that you would want to disable.  If you are running the SSID wide open with no splash page and in NAT mode with no content filtering, then something further upstream must be redirecting the clients to the AV scan other than the MR33.   ... View more