Most of the SonicWALL VPN issues are related to some settings being changed in the network driver. Or because of users not installing regular windows patches or drivers recommended. Or not being restarted for quite a long. One of the commands I feel very handy and has resolved most of the issues on Windows 7 and Windows 10 OS is netsh int ip reset. Please run the same in CMD admin mode and a restart is usually not required. You can run this command without fear it does not change anything major or introduce any new issues, till now with my experience. It is a super magic command which helps to resolve most of the VPN issues. And the if still unresolved you can try reinstalling the VPN client. It works good for most models and versions of SonicWALL client.