Dear @jgwentworth,

Try changing the following parameters on the TL-R600VPN router:

• Exchange Mode: Aggressive Mode
• Negotiation Mode: Responder Mode
• DPD: Disabled if the SonicWALL doesn't have this option configured.

Additional info for checking:

• Ensure the Remote Gateway is configured correctly on both routers.
• Ensure the Pre-shared key matches on both routers.
• If the WAN IP address is Public IP but not Static, it's suggested to configure DDNS on the router. 
• If the VPN router is behind NAT, the ports UDP 500 & UDP 4500 need to be open on the NAT device.
• Check the current firmware version of the TL-R600VPN, ensure it's up to date.
• Check the System Log for troubleshooting.