Usually service providers have in their portfolio MPLS L3 VPNs connections with alternative technologies of interconnection to provide customer site redundancy and resiliency.
The main connection may be xDSL based or Fiber and the backup xDSL (if the main connection is based on Fiber) or 4G. The protocol used may be different depending on the SP and your contract (static routes are very popular).
4G backups are not always the best option to transport voice traffic. Due to that, your service may be degraded when your site is using the backup connection.
I want to use a normal public internet traffic. I'ver seen this company that has this "internet backpack" with multiple SIM, they use them to increase bandwith and stability at the same time. I figure that the traffic of their backpack pass through their server to being merged, so my question is: there is any usable protocol to make that, or the protocol is proprietary and hide from the public?