From my understanding, and fellow Meraki Nerds please correct me if I am wrong, when you select "Default Route" all traffic will go through that route. If that route is down, it is unable to connect to the hub to send through the traffic.
IF, you untick the "Default Route" it sends traffic via a "Split Tunnel" (Selected network subnets connects via the selected VPN). This allows internet traffic to go through the Meraki Device's 4G connection.
Because you have the "Default Route" selected, all traffic is trying to go via the VPN link. Hope this makes sense...
In our environment, we have to select "Default Route" due to a proxy client. But if we do not have the proxy installed for a particular site, we are able to have the MX device in Split Tunnel mode. This way, if the site loses connectivity to the hub, they are still able to have internet access. They will reconnect to network services once the VPN connection re-establishes.