Which router to balance wan and 4g usb?

I am looking to connect a Netgear Nighthawk M1 on AT&T lte via wan and a Inseego Mifi 8000 on Sprint lte via USB to a single router and have some sort of load balance (even if round Robin style).

I see that many of the Gl.Inet routers will allow usb data tether but with they also allow simultaneous wan and usb data input and utilize or load balance between the 2 connections?

By default it does failover. But you can try to configure as load balance.

Checkout mwan3 config [OpenWrt Wiki] mwan3 (Multi WAN load balancing/failover)

mwan3 is used to do the loadbalance and failover. The core parameter is the “metric” in network interfaces.