We also hit this limitation. Perhaps following will help somebody:
- As you can't change default GRE Tunnel you need to create a new one with a higher MTU and let you WLAN use this new tunnel.
- Via CLI you can view/configure the MTU on AP's with "get mtu ..." and "set mtu ...".
- If you want to change the MTU on the Smartzone interface you need to open a ticket so Ruckus support can change it for you via remote session.
I requested feature request to do this via Smartzone