Is there a way to have the fastest connection automatically selected when using connections and failover?
For example, if I have a modem tethered to one lte carrier and the internal modem connected to a different lte carrier can the software use whichever is the fastest one?
Sorry I don’t think it is possible.
There seems to be no way to do that, at least with all connections enabled. There is no silver bullet, especially since I suspect that over-preferring wired WANs is a fundamental flaw in OpenWRT or Linux 
You need a script that evaluates the connection quality while turning each connection on / off and determines the fastest connection.
This is a fairly flawed method, so I don’t think it will be officially supported.