Check and change MTU size on the SMA, must be done by SSH console using this command:
- Connect to console or SSH
-Run #ifconfig eth0, and you can see the actual value of the MTU.

- #ifconfig eth0 mtu 1500, (where 1500 it's the value for the MTU).

To enabling and obtaining SSH access, please follow the document below:
https://support.sonicwall.com/sonicwall-secure-mobile-access/kb/sw2500