I personally would go for something like the: hAP ac³ LTE6 kit
It is not supported yet by OpenWRT but RouterOS is not that bad. Since the router is based on the IPQ4019 platform, with some tinkering it can be supported.
Personally, I have switched to Alcatel HH71VM units since they can be purchased for cheap and have a micro usb serial port. That said, they are using the RealTek RTL8197FS-VE5 which is not officially supported yet. Still needs some love according to this post: Any plans for Realtek SOC support? - #22 by gaspare