First I try this method:
https://forum.openwrt.org/t/mikrotik-r11e-lte-us-with-openwrt/16604/4
the wan interface won't came up.
Then I try this:
https://forum.openwrt.org/t/installing-openwrt-on-mikrotik-wap-r-rbwapr-2nd/15423/119
With this the WAN interface came up but no internet access, and the interface got IP like 192.168.1.139 ...