Hello,
can anybody recomend a modem, which i could connect to HEX S via USB and which could bound external (public) IP to routerboard LTE interface. Now i have ZTE MF823, which gets public IP form ISP, but makes nat and gives to my routerboard 192.168.35.0 IP.

I have no experience of ZTE but it should be possible to login on the router (192.168.35.1) and enable pass through mode in some way.

nope, it is simple LTE stick, so you can not login anythere. now i have ordered SXT LTE kit, witch i think has R11e modem and supports LTE passthrough and will try it