SonicWall allow users to change the default MSS for VPN traffic with enabling the option do not adjust TCP MSS option for VPN traffic in the diag page, then MSS should be determined by the end points in the TCP three-way handshake.
Get the Hidden Diag page:
Log in to SonicWall, and instead of "main.html" use "diag.html" (for example when the device has an IP address 192.168.1.1 go to https://192.168.1.1/diag.html).
Then click on "Internal Settings" and scroll down to VPN settings.
3: Check Do not adjust TCP-MSS (Minimum Segment Size) option for VPN traffic.
Click Accept at the top of the page and click close.