I disagree about arguments regarding ethics or usefulness of flashing more featureful firmware. The Sprint router is given to the customer and not lent. Ownership and device fees are spelled out in the device manual.
Other than removing Bridge and Repeater modes, there was no need for Sprint to disable the features they did in order to make WiFi Calling the top priority for QoS.
Flashing Asus or aftermarket firmware doesn't stop my device from connecting to the router for Wifi Calling and Data in order to reduce traffic on Sprint's inadequate network. Just make Wifi Calling traffic top priority on your newly enabled QoS page. Google tells me it's on UDP port 4500, and you can check netstat logs during a Wifi Call to confirm this.