I’m speaking from experience here. I’ve done exactly what you are trying to do.
http://www.mobile-stream.com/beta/openwrt/easytether-usb-openssl_0.8.9-4_openwrt-18.06.2.zip
The file you want is in ./*/ar71xx/generic
(Always grab the latest version from Download EasyTether drivers - EasyTether for Mac OS X, Windows, Ubuntu, Fedora and game consoles. The one you want is “OpenWRT” w/ openssl)
Btw, there is a way to setup tap-easytether as a fallback connection. The way I have my slate set up is I use Ethernet (WAN) as the top priority, followed by Wi-Fi repeater, followed by EasyTether. I did this by setting up a separate interface instead of replacing the existing WAN interface in the network file. Set the metric for Ethernet to 10, set the metric for Wi-Fi repeater to 20, and set the metric for EasyTether to 30.
My router is currently “rented” out of state atm and I won’t be able to see it’s configuration for a month or so until I get it back, but when I get it back, I can walk you through the steps to set up fallback connections, if you are interested.
Hope this helps!
P.S. Btw, if you can figure out why easytether stops working after a while, let me know. It’s been driving me nuts to have to unplug and replug the phone back and forth into the router to start back up the internet for easytether.