Hello,
when pluging the Huawei 3531s-2 in the usb port of the RB951ui 2hnd,
upon "system resources usb print"
you can see it but when the port remains unknown.

Is there any solution?

Hello,
It is Huawei e3531 s-2

See here the list of the supported devices.
Scroll down to the supporting 3 g modems

http://wiki.mikrotik.com/wiki/Supported_Hardware

Is there any resolve to this issue? I also have the E3531 and it shows up as an LTE interface but doesn't connect to the internet or show up as a port even though it appears in the resources as a usb device.

Is there any resolve to this issue? I also have the E3531 and it shows up as an LTE interface but doesn't connect to the internet or show up as a port even though it appears in the resources as a usb device.

Have You configured all necessary things for LTE interface? People mostly don't know that must be added DHCP client for that interface.

See here the list of the supported devices.
Scroll down to the supporting 3 g modems

http://wiki.mikrotik.com/wiki/Supported_Hardware

Even if it is not on the supported hardware list it could work as well.

I`m testing with a Huawei E3531s-2 on a CRS125-24G-1S-2HnD-IN and the Huawei seems to be working if you plug the stick in and poweroff/poweron the unit. Reboot is not sufficient.

The Huawei seems to be acting as a NAT router using 192.168.8.x subnet on the USB side of the dongle and the public provider IP on the wireless side of the dongle.

I want to use the public ip on Mikrotik so i need to find out how

I`m testing with a Huawei E3531s-2 on a CRS125-24G-1S-2HnD-IN and the Huawei seems to be working if you plug the stick in and poweroff/poweron the unit. Reboot is not sufficient.

The Huawei seems to be acting as a NAT router using 192.168.8.x subnet on the USB side of the dongle and the public provider IP on the wireless side of the dongle.

I want to use the public ip on Mikrotik so i need to find out how

Hello,
Are you able to ping your public IP from the wild internet?

Currently this sim card is not provided with a public IP adres. But another sim card in a Cisco Router with 3G interface is publicly reachable and equipped a public IP