you dont have to disable dhcp on the ZTE. even if you use ippassthrough, you can't disable the dhcp on the bridge interface.
So you must set dhcp client on the wan on openwrt.
If ZTE router ip have 2 interface, check which one they are, example if there is no 2nd interface, wwhit ippassthroug you shouldn't be able to reach it after you set ippassthrough, so check better.
Once you have the ZTE ip range, use a different one for openwrt lan.
example ZTE= 192.168.0.1, you set 192.168.1.1 for your openwrt lan.
Also better set DNS on wan interface of your openwrt.
let me know if it works or you need more info.