The following Table might be what you are looking for:
SONICWALL-FIREWALL-IP-STATISTICS-MIB:sonicSAStatTable - 184.108.40.206.4.1.87220.127.116.11.1.1
I advise you to do a MIB walk on your device to figure out which MIBs are supported by this model.
I have this issue either.
The problem is when the vpn goes down, the oid number changes.