Sonicwall is using 10.0.0.0/8 for its LAN network. That conflicts with the tunnel network that you have chosen. So the server pfSense will be confused about where 10.0.8.0/30 actually is.
Either:
At main office, Sonicwall will need a route to 192.168.0.0/16 through pfSense LAN IP 10.1.1.253 - this will allow systems on main office LAN to send packets to your client end using their default gateway (Sonicwall) which will redirect them to pfSense.