The following Table might be what you are looking for:
SONICWALL-FIREWALL-IP-STATISTICS-MIB:sonicSAStatTable - 18.104.22.168.4.1.8722.214.171.124.1.1
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.