There is a problem with the 3g modem.
When using a modem Huawei E-1550 modem and prescribing APN successfully assigned IP address from the network 10.250.18.x
when you use 3G modem Huawei E-303. Router OS 6 rc06 RB751-2nd not assigned an address from the network 10.250.18.x. Uses the same APN.
what is problem?